lib/, src/: Simplify allocation of buffer

lib/, src/: Simplify allocation of buffer

getgroups(0, NULL) returns the number of groups, so that we can allocate
at once.  This might fail if there's a race and the number of users
grows while we're allocating, but if that happens, failing is probably a
good thing to do.

There was some comment saying it doesn't work on some systems, but
according to gnulib, that's only NeXTstep 3.2, which we don't support.

Link: <https://www.gnu.org/software/gnulib/manual/html_node/getgroups.html>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>