network-manager: fix DHCP clients detection.

If either dhclient or dhcpcd are selected, network-manager is now
configured with the proper paths.

This allows 'dhcp=dhclient' or 'dhcp=dhcpcd' to be set in
/etc/NetworkManager/NetworkManager.conf.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>