Add if_nametospec helper to decode and interface name such as bge1:2 into

Add if_nametospec helper to decode and interface name such as bge1:2 into
device name, lun, ppa and driver name.
Use this function when discovering addresesses and when they arrive
via the kernel listening socket so they can be assigned to the correct
physical interface.

This change does also stop `dhcpcd eth0:1` working on Linux, but is needed
for basic Solaris support.