use read syscall directly to get character

Due to go std lib uses poller for os.File introducing in this commit:
golang/go@c05b06a
There are two changes to watch out:
1. os.File.Fd will always return a blocking fd except on bsd.
2. os.File.Read won't return EAGAIN error for nonblocking fd.

So
For 1, we just get tty's fd in advance and then set its block mode.
For 2, we use read syscall directly to get what we wanted error(EAGAIN).

Fix issue #910.

Signed-off-by: Tw <tw19881113@gmail.com>

src/tui/light.go

@@ -251,8 +251,9 @@ func (r *LightRenderer) updateTerminalSize() {
 
 func (r *LightRenderer) getch(nonblock bool) (int, bool) {
 	b := make([]byte, 1)
+	fd := r.fd()
 	util.SetNonblock(r.ttyin, nonblock)
-	_, err := r.ttyin.Read(b)
+	_, err := util.Read(fd, b)
 	if err != nil {
 		return 0, false
 	}

src/util/util_unix.go

@@ -26,3 +26,8 @@ func IsWindows() bool {
 func SetNonblock(file *os.File, nonblock bool) {
 	syscall.SetNonblock(int(file.Fd()), nonblock)
 }
+
+// Read executes syscall.Read on file descriptor
+func Read(fd int, b []byte) (int, error) {
+	return syscall.Read(int(fd), b)
+}

src/util/util_windows.go

@@ -32,3 +32,8 @@ func IsWindows() bool {
 func SetNonblock(file *os.File, nonblock bool) {
 	syscall.SetNonblock(syscall.Handle(file.Fd()), nonblock)
 }
+
+// Read executes syscall.Read on file descriptor
+func Read(fd int, b []byte) (int, error) {
+	return syscall.Read(syscall.Handle(fd), b)
+}