remove ( simple_command & ) optimisation (re: e3d7bf1)

I didn't trust this back in e3d7bf1 (which disabled it for
interactive shells) and I trust it less now. In af6a32d/6b380572,
this was also disabled for virtual subshells as it caused program
flow corruption there. Now, on macOS 10.14.6, a crash occurs when
repeatedly running a command with this optimisation:

$ ksh -c 'for((i=0;i<100;i++));do print -n "$i ";(sleep 1&);done'
0 1 2 3 4 5 6 7 Illegal instruction

Oddly enough it seems that I can only reproduce this crash on macOS
-- not on Linux, OpenBSD, or Solaris. It could be a macOS bug,
particularly given the odd message in the stack trace below.

I've had enough, though. Out it comes. Things now work fine, the
reproducer is fixed on macOS, and it didn't optimise much anyway.

The double-fork issue discussed in e3d7bf1 remains.
________
For future reference, here's an lldb debugger session with a stack
trace. It crashes on calling calloc() (via sh_calloc(), via
sh_newof()) in jobsave_create(). This is not an invalid pointer
problem as we're allocating new memory, so it does look like an OS
bug. The "BUG IN CLIENT OF LIBPLATFORM" message is interesting.

$ lldb -- arch/*/bin/ksh -c 'for((i=0;i<100;i++));do print -n "$i ";(sleep 1&);done'
(lldb) target create "arch/darwin.i386-64/bin/ksh"
Current executable set to 'arch/darwin.i386-64/bin/ksh' (x86_64).
(lldb) settings set -- target.run-args  "-c" "for((i=0;i<100;i++));do print -n \"$i \";(sleep 1&);done"
(lldb) run
error: shell expansion failed (reason: lldb-argdumper exited with error 2). consider launching with 'process launch'.
(lldb) process launch
Process 35038 launched: '/usr/local/src/ksh93/ksh/arch/darwin.i386-64/bin/ksh' (x86_64)
0 1 2 3 4 5 6 7 8 9 Process 35038 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x00007fff70deb1c2 libsystem_platform.dylib`_os_unfair_lock_recursive_abort + 23
libsystem_platform.dylib`_os_unfair_lock_recursive_abort:
->  0x7fff70deb1c2 <+23>: ud2

libsystem_platform.dylib`_os_unfair_lock_unowned_abort:
    0x7fff70deb1c4 <+0>:  movl   %edi, %eax
    0x7fff70deb1c6 <+2>:  leaq   0x1a8a(%rip), %rcx        ; "BUG IN CLIENT OF LIBPLATFORM: Unlock of an os_unfair_lock not owned by current thread"
    0x7fff70deb1cd <+9>:  movq   %rcx, 0x361cb16c(%rip)    ; gCRAnnotations + 8
Target 0: (ksh) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
  * frame #0: 0x00007fff70deb1c2 libsystem_platform.dylib`_os_unfair_lock_recursive_abort + 23
    frame #1: 0x00007fff70de7c9a libsystem_platform.dylib`_os_unfair_lock_lock_slow + 239
    frame #2: 0x00007fff70daa3bd libsystem_malloc.dylib`tiny_malloc_should_clear + 188
    frame #3: 0x00007fff70daa20f libsystem_malloc.dylib`szone_malloc_should_clear + 66
    frame #4: 0x00007fff70dab444 libsystem_malloc.dylib`malloc_zone_calloc + 99
    frame #5: 0x00007fff70dab3c4 libsystem_malloc.dylib`calloc + 30
    frame #6: 0x000000010003fa5d ksh`sh_calloc(nmemb=1, size=16) at init.c:264:13
    frame #7: 0x000000010004f8a6 ksh`jobsave_create(pid=35055) at jobs.c:272:8
    frame #8: 0x000000010004ed42 ksh`job_reap(sig=20) at jobs.c:363:9
    frame #9: 0x000000010004ff6f ksh`job_waitsafe(sig=20) at jobs.c:511:3
    frame #10: 0x00007fff70de9b5d libsystem_platform.dylib`_sigtramp + 29
    frame #11: 0x00007fff70d39ac4 libsystem_kernel.dylib`__fork + 12
    frame #12: 0x00007fff70c57d80 libsystem_c.dylib`fork + 17
    frame #13: 0x000000010009590d ksh`sh_exec(t=0x0000000101005d30, flags=4) at xec.c:1883:16
    frame #14: 0x0000000100096013 ksh`sh_exec(t=0x0000000101005d30, flags=4) at xec.c:2019:4
    frame #15: 0x0000000100096c4f ksh`sh_exec(t=0x0000000101005a40, flags=5) at xec.c:2213:9
    frame #16: 0x0000000100096013 ksh`sh_exec(t=0x0000000101005a40, flags=5) at xec.c:2019:4
    frame #17: 0x000000010001c23f ksh`exfile(iop=0x0000000100405750, fno=-1) at main.c:603:4
    frame #18: 0x000000010001b23c ksh`sh_main(ac=3, av=0x00007ffeefbff4f0, userinit=0x0000000000000000) at main.c:365:2
    frame #19: 0x0000000100000776 ksh`main(argc=3, argv=0x00007ffeefbff4f0) at pmain.c:45:9
    frame #20: 0x00007fff70bfe3d5 libdyld.dylib`start + 1

src/cmd/ksh93/sh/xec.c

@@ -1868,24 +1868,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 					sh.exitval -= 128;
 				sh_done(0);
 			}
-			else if(((type=t->par.partre->tre.tretyp)&FAMP) && ((type&COMMSK)==TFORK)
-			&& !job.jobcontrol && !sh.subshell)
-			{
-				/* Optimize '( simple_command & )' */
-				pid_t	pid;
-				sfsync(NIL(Sfio_t*));
-				while((pid=fork())< 0)
-					_sh_fork(pid,0,0);
-				if(pid==0)
-				{
-					sh.current_pid = getpid();
-					sh_reseed_rand((struct rand*)RANDNOD->nvfun);
-					sh.realsubshell++;
-					sh_exec(t->par.partre,flags);
-					sh.st.trapcom[0]=0;
-					sh_done(0);
-				}
-			}
 			else
 				sh_subshell(t->par.partre,flags,0);
 			break;

src/cmd/ksh93/tests/variables.sh

@@ -69,6 +69,7 @@ done
 (( rand1 == rand2 )) && err_exit "Test 3: \$RANDOM seed in subshell doesn't change" \
 	"(both results are $rand1)"
 # $RANDOM should be reseeded for the ( simple_command & ) optimization
+# (which was removed on 2022-06-13, but let's keep the test)
 for((i=0; i<N; i++))
 do	( echo $RANDOM & ) >|r1
 	( echo $RANDOM & ) >|r2