Hoist PTP socket handling, drop privileges on OpenBSD

Hoist PTP socket handling, drop privileges on OpenBSD

bind(2)ing ports below 1024 is the only privileged operation NQPTP does.

Move its code up in main() before shared memory handling such that root
privileges can be dropped immediately after it;  no currently supported
system does that, thus this should be a NOOP.

Do so on OpenBSD where shm_open(3) does not allow access to shared memory
objects by multiple UIDs, i.e. to communicate, shairport-sync and NQPTP
must create them and run as the very same user.

OpenBSD's official audio/shairport-sync user provides an rc.d(8) daemon
script that runs as `_shairport` user.