nspawn: fix exit code for --help and --version (#4609)

Commit b0067625 inverted the initial exit code which is relevant for --help and
--version without a particular reason.  For these special options, parse_argv()
returns 0 so that our main() immediately skips to the end without adjusting
"ret". Otherwise, if an actual container is being started, ret is set on error
in run(), which still provides the "non-zero exit on error" behaviour.

Fixes #4605.
(cherry picked from commit cfed63f6)