terminal-util: introduce openpt_allocate()

Allocating a pty is done in a couple of places so let's introduce a new helper
which does the job.

Also the new function, as well as openpt_in_namespace(), returns both pty
master and slave so the callers don't need to know about the pty slave
allocation details.

For the same reasons machine_openpt() prototype has also been changed to return
both pty master and slave so callers don't need to allocate a pty slave which
might be in a different namespace.

Finally openpt_in_namespace() has been renamed into
openpt_allocate_in_namespace().