simplify mgetgroups() and avoid -Wsign-compare warnings

simplify mgetgroups() and avoid -Wsign-compare warnings

* gl/lib/mgetgroups.c: Avoid -Wsign-compare warning by using unsigned
types for the parameters of the new function realloc_groupbuf().
mgetgroups() was refactored to use this function rather than
explicitly allocating and copying from automatic storage itself.
* src/group-list.c: Use int rather than size_t as variable is
used in signed comparisons.
* src/id.c: ditto.