mgetgroups: new module, taken from coreutils

mgetgroups: new module, taken from coreutils

Wrapper function that makes using getgroups/getugroups easier to use.
As part of the move from coreutils, convert GETGROUPS_T to gid_t,
and allow mgetgroups(NULL,getegid(),&list) as a way to ensure that
the effective gid is in the list.

* modules/mgetgroups: New file.
* lib/mgetgroups.h: Likewise.
* lib/mgetgroups.c (mgetgroups): Likewise.
* m4/mgetgroups.m4 (gl_MGETGROUPS): Likewise.
* MODULES.html.sh (Users and groups): Mention it.

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