Added core.sys.posix.grp (from Posix <grp.h>).

No Solaris declarations for now.
Added core.sys.posix.grp to all make files.
Added XSI prototypes.
Put the optional prototypes into per-platform version statements.
Added nothrow...
Made the void() functions trusted.
Merge pull request #373 from s-ludwig/master

Add core.sys.posix.grp to complement core.sys.posix.pwd