group-member: omit unnecessary dependencies

group-member: omit unnecessary dependencies

This is for Emacs, which has its own allocator and where we
don't want to use xalloc.
* lib/group-member.c: Include xalloc-oversized.h, not xalloc.h,
since we no longer use xmalloc.  Do not include stdbool.h, since
the changes below happen to remove the only use of bool.
(GROUPBUF_SIZE): New constant.
(struct group_info): Remove n_groups member.  Add groupbuf member.
This lets us get the groups without using malloc, usually.
(free_group_info, get_group_info): Adjust to this.
(get_group_info): Return the number of groups found, or -1 on error.
Use plain malloc not xmalloc, and treat its failure as if there
are no groups, as the user already loses in case of error.
(group_member): Simplify, based on changes to get_group_info.
* modules/group-member (Depends-on): Remove dependencies on
xalloc and stdbool.  Add dependency on xalloc-oversized.