getgroups: avoid calling exit

getgroups: avoid calling exit

rpl_getgroups should be a library function, comparable to glibc.

* modules/getgroups (Depends-on): Add malloc-posix and unistd,
drop xalloc.
* modules/getgroups-tests (Depends-on, Makefile.am): Drop unneeded
dependencies.
* lib/getgroups.c (rpl_getgroups): Fail with ENOMEM rather than
exiting, in the rare case of malloc failure.

Signed-off-by: Eric Blake <ebb9@byu.net>