Functions: Implement our own which logic in have() function.

On distributions sbin is usually not in the users PATH and we fail
to detect dhcpd(3) and/or dnsmasq.

The have() function now uses our own which implmentation and
adds both /sbin and /usr/sbin when reading PATH. It will now return
the the full path when found or None if not.

Also update NetConf.py to use the updated have() function.
Support both dhcpd and dhcpd3.