Fix #2831: Towards a more complete posixlib unistd.scala

[LeeTibbert]

Fix #2831. posixlib unistd.scala is more capable and useful now. It is almost complete.

• A number of A number of _CS, _PC, _SC symbolic constants were not implemented
  because they are not defined on Linux, macOS, or both. Windows
  appears to support this subset.

  A few other constants or methods were not implemented.

   // _POSIX2_VERSION was not implemented
  
   // pid_t setpgrp(void); // [OB XSI], not implemented
  

  See the file for the complete list.

• Two methods were deprecated in order to match the POSIX spec.
  These need a release note but the 0.5.0 release file is awaiting merge, hence locked.
  I will create an Issue so that these deprecations do not get lost.

    @deprecated(
      "Not POSIX, subject to complete removal in the future.", 
      since = "posixlib 0.5.0"
   )
   def sethostname(name: CString, len: CSize): CInt = extern
  
   @deprecated(
     "Removed in POSIX.1-2008. Consider posix_spawn().",
     "posixlib 0.5.0"
   )
   def vfork(): CInt = extern
  

• An method deprecated in Scala Native 0.4.5 was not removed because, IMO,
  a sufficient interval has not yet passed.

2022-10-19 For the enquiring mind, the deprecated items are now recorded in the
0.5.0 Changelog (PR #2924)

[WojciechMazur]

That's a lot of new bindings and improvements. Thank you!