Make hammer.py work with FreeBSD 14

hammer.py should have worked out of the box with FreeBSD 14, but there seems to be a problem with the service postgresql onestart command starting the postgresql server and then complaining that there is already a postgresql server started which itself did.

On top of that, there is another irrational issue about the execute call that runs that command which seems to freeze, possibly because it exits before communicate(), though that theoretically shouldn't be a problem. In fact, communicate() instead of wait() or poll() is what solves this kind of problem.

This issue is about making these situations less error-prone for FreeBSD 14 and future versions.