Change getgroups to use GETGROUPS_T as the type of the group array returned,

Change getgroups to use GETGROUPS_T as the type of the group array returned,
as calculated by the configure macro AC_TYPE_GETGROUPS.  Without that, it
doesn't work properly on systems like sunos 4 where gid_t is defined to
be an unsigned short but getgroups is defined to return an array of integers.