net-mgmt/p0f: add rc script

Added rc script to run p0f in daemon mode as as unprivileged user.
That is useful to provide access to p0f API via unix socket for
various clients (e.g. anti-spam filters like rspamd, haraka-plugin-p0f,
etc.).

PR: 240712
Submitted by: Alexander Moisseev <moiseev@mezonplus.ru>