net/rsync: Fix stealth dependency on Python

net/rsync includes a Python script installed as sbin/rrsync
without declaring a dependency on Python.

Adds a port option, PYTHON, that controls whether rrsync is
installed and adds the necessary dependency and remove the
pkg-message who advises that Python might be needed.

By default this option is off (updating the package will remove rrsync).

PR: 286073
Reported by: Christian Ullrich <chris@chrullrich.net>