lib/, src/: Replace redundant checks by actual error handling

lib/, src/: Replace redundant checks by actual error handling

setgroups(2) already performs a test to check if the number of groups is
too large.  Don't do that ourselves, and also don't do it for every
iteration.  Just let setgroups(2) do it once.

Instead of our check, let's report errors from setgroups(2).

Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>