core: add Timestamping= option for socket units

core: add Timestamping= option for socket units

This adds a way to control SO_TIMESTAMP/SO_TIMESTAMPNS socket options
for sockets PID 1 binds to.

This is useful in journald so that we get proper timestamps even for
ingress log messages that are submitted before journald is running.

We recently turned on packet info metadata from PID 1 for these sockets,
but the timestamping info was still missing. Let's correct that.