This commit is based on a patch by Roy Marples <roy@marples.name>

Refine the detection logic of the dhcp client to not require a installed dhcp
client and be more backwards compatible:
1.) If no argument is given, first search for dhclient, then dhcpcd. If none is
    found, fall back to /sbin/dhclient (old behaviour)
2.) If either dhclient or dhcpcd is used as argument to --with-dhcp-client,
    search for the binary and the correct version. Fail if not found.
3.) If a complete path is given, do no search for the binary.

Finally, check if the backend is either "dhclient" or "dhcpcd" and fail
otherwise.

git-svn-id: http://svn-archive.gnome.org/svn/NetworkManager/trunk@4002 4912f4e0-d625-0410-9fb7-b9a5a253dbdc