Check for required icon themes.

We can use either gnome or mate icon theme but we need one.
Add icon theme requirements
Functions: Implement our own which logic in have() function.

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

The have() function now returns the full path or None.

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