Add fixes for OpenWRT DHCP modes 'client' and 'none'.

This pull request applies some minor fixes to the DHCP modes 'client' and 'none,' namely to ensure that interfaces are properly removed from the client bridge and only receive an IP address if manually configured in mode 'none,' and so that udhcpc is executed with the correct parameters in mode 'client.' These changes allow for proper manual gateway configuration and for meshing over ethernet to function correctly. This should fix https://github.com/opentechinstitute/luci-commotion/issues/173 and 
https://github.com/opentechinstitute/luci-commotion/issues/174.

Additionally, the unused uci_state parameter 'lease' is removed by this commit.

To test:
1. Configure two Commotion nodes through the setup wizard, and then under the 'additional network interfaces' page, choose to have them mesh over ethernet. Set one with the address 192.168.55.1 and the other with 192.168.55.2, netmask 255.255.255.0.
2. Connect the two nodes together via ethernet, and ensure that routes for the addresses specified above show up on each of their status pages.
3. Under additional interfaces on one node, deselect the option to mesh over ethernet and then save. Make sure you can still connect to the node via its access point.
4. On that same node, under additional interfaces, select for its interface to always attempt to get a DHCP lease and save. Connect it to an upstream gateway over ethernet, and make sure it gets a lease.
5. Reboot the node, and then make sure that you do not get a lease when connecting to it via ethernet.