Add four utility functions related to struct grp processing modeled in-part

Add four utility functions related to struct grp processing modeled in-part
after similar calls related to struct pwd in libutil/pw_util.c:
  - gr_equal()
    Perform a deep comparison of two struct grp's.  It does a thorough, yet
    unoptimized comparison of all the members regardless of order.

  - gr_make()
    Create a string (see group(5)) from a struct grp.

  - gr_dup()
    Duplicate a struct grp.  Returns a value that is a single contiguous
    block of memory.

  - gr_scan()
    Create a struct grp from a string (as produced by gr_make()).

MFC after:	3 weeks