Log Message:
bitlbee: Always create the ${pidfile} directory and improve rc.d script

- At least on NetBSD every files on /var/run are removed at every
  boot by mountcritlocal rc.d script. Add a bitlbee_precmd() function
  to always check that a directory for the ${pidfile} exists and
  create it if needed.
- Check if /etc/rc.subr exists before source-ing it.
- Introduce BITLBEE_USER and BITLBEE_GROUP variables and reuse them to avoid
  hardcoding `bitlbee' and `nobody' (NFCI)

Discussed with and suggestions from <tonio>, thanks!