setuidgid: set all groups, not just the primary one; mgetgroups: new module

setuidgid: set all groups, not just the primary one;  mgetgroups: new module

I wanted to use the xgetgroups function from id.c, so factored it out
and made it into a non-exiting function (hence the "m" prefix rather than "x").
* src/setuidgid.c (main): Use mgetgroups. Include "mgetgroups.h".

* src/id.c (xgetgroups): Remove function.
Include "mgetgroups.h".
(print_group_list): Use mgetgroups, not xgetgroups.

* gl/modules/mgetgroups: New module.
* gl/lib/mgetgroups.c: New file.  mgetgroups is derived from
id.c's xgetgroups function.
* bootstrap.conf (gnulib_modules): Add mgetgroups.
* gl/m4/mgetgroups.m4: New file.
* gl/lib/mgetgroups.h: New file.