mgetgroups: relax license to LGPLv2+

mgetgroups: relax license to LGPLv2+

getgrouplist is part of glibc and LGPLv2+; the mgetgroups module
is merely an ease-of-use wrapper around this interface.  There's
nothing in the algorithm worth protecting by the more-restrictive
GPLv3+.  See also this thread.
https://lists.gnu.org/archive/html/bug-gnulib/2013-05/msg00092.html

* modules/getugroups (License): Change from GPLv3+.
* modules/mgetgroups (License): Likewise.
* modules/getgroups (License): Change from LGPLv3+.

Signed-off-by: Eric Blake <eblake@redhat.com>