core: allow Type=oneshot services to have ExecReload

core: allow Type=oneshot services to have ExecReload

Use cases:

 * iptables.service – atomically reload rules without having to flush
   them beforehand (which may leave the system insecure if reload fails)

 * rpc-nfsd.service – reexport filesystems after /etc/exports update
   without completely stopping and restarting nfsd

(In both cases, the actual service is provided by a kernel module and
does not have any associated user-space processes, thus Type=oneshot.)