Make iface completion work better

This commit contains several improvements to the interface completion in
ovs-vsctl-bashcomp.bash.  Firstly, it now separately completes "system"
interfaces and the interfaces from the database, allowing
add-bond/add-port to use system interfaces and del-port/iface-to-br to
use the interfaces from the db.  Secondly, the add-port command now
completes new ports as interfaces.  Finally, system iface completion now
attempts to use the "ip" command from iproute2 on Linux systems where
the ifconfig command is deprecated.
Make _ovs_vsctl_bashcomp_localopt work with = options

The _ovs_vsctl_bashcomp localopt function now works correctly with
options of the form <option>= which take a parameter and makes sure not
to add a space after the argument completion in those cases.ñ