Track SSID in leases file and only consider leases from the current SSID when

Track SSID in leases file and only consider leases from the current SSID when
starting up dhclient on wifi interfaces. In theory will preserve leases from
other SSID's and speed up obtaining a lease by not wasting time attempting
to re-acquire a lease from a different SSID.

Experimental feature from n2k16 needing some real world testing before g2k16.