Get rid of the following gcc format warnings by simplifying the

Get rid of the following gcc format warnings by	simplifying the
getgroups implementation:

  mg.c: In function Perl_magic_get':
  mg.c:1008: warning: long unsigned int format, gid_t arg (arg 3)
  mg.c:1014: warning: long unsigned int format, gid_t arg (arg 3)
  mg.c:1025: warning: long unsigned int format, unsigned int arg (arg 3)

Since we already cast the numeric Gid_t values to an IV it should not
be too risky to also cast the Group_t values.  Converting these values
with Gid_t_f wasn't quite right anyway.

p4raw-id: //depot/perl@26786