terminal/subterm: use usec_t instead of "unsigned long"

Avoid hard-coding "unsigned long" and use the usec_t type defined in
src/shared.
barrier: convert msecs to usecs in test-code

Avoid using msecs in favor of usec_t. This is more consistent with the
other parts of systemd and avoids the confusion between msec and usec. We
always use usecs, end of story.