-n switch for nslcd (prevents process from forking)
-n switch for pynslcd
Document -n, --nofork option
Implement an option to run in the foreground

This introduces a -n, --nofork option that skips the deamonising step on
start-up. This may be required for running nslcd from upstart.

See:
https://bugs.launchpad.net/bugs/806761