Improve os.date / strftime

[bvisness]

The implementation of os.date didn't support many flags that I was used to from other systems. This new implementation supports many more of the flags from the C standard for strftime, plus some utilities borrowed from other strftime implementations such as glibc and Ruby.

The overall changes are:

• Added the following flags unchanged from the C spec: %n, %t, %j, %e, %D, %r, %R, %T, %G, %g, %V
• %x was just wrong; it formatted a time as if it was a date. It has now been defined to be the same as %D.
• Added:
  • %-m: trimmed month
  • %-d: trimmed day of month

I see that this flag-scanning code is also used by strGsubStr. Please do let me know if my changes are unacceptable because of that!

[LittleToonCat]

Would like to know when this will get merged. My Lua code uses os.date but %a is not formatted correctly in the current gopher-lua codebase and will always result in "mon" instead "Mon" or any other correct day for that matter. I was going to submit a fix for that bug myself, but then this PR came along and now I am just waiting for it to get merged.