Put the code to get a unit number from a device name into a common

Put the code to get a unit number from a device name into a common
routine, and use it both on HP-UX and other DLPI systems; this means
that, in case there is ever a network device on HP-UX with a number in
the device type name, we'll properly extract the unit number (i.e.,
we'll extract the last number from the name, not the first number) - I
don't think that'll ever happen, but putting it into a common routine is
cleaner in any case.