core/namespace: allow using ProtectSubset=pid and ProtectHostname=true together

core/namespace: allow using ProtectSubset=pid and ProtectHostname=true together

If a service requests both ProtectSubset=pid and ProtectHostname=true
then it will currently fail to start. The ProcSubset=pid option
instructs systemd to mount procfs for the service with subset=pid which
hides all entries other than /proc/<pid>. Consequently trying to
interact with the two files /proc/sys/kernel/{hostname,domainname}
covered by ProtectHostname=true will fail.

Fix this by only performing this check when ProtectSubset=pid is not
requested. Essentially ProtectSubset=pid implies/provides
ProtectHostname=true.

(cherry picked from commit 1361f015773e3b4d74e382edf1565f3315a3396b)