added functions to pad mac strings for net.ParseMAC()

[chrismarget]

On MacOS the MAC address strings returned by the ARP library are not directly consumable by the standard library's net.ParseMAC() function because the values aren't printed with a consistent number of digits.

This change adds a function that left pads the chunks between delimiters. If the delimiter is ":" or "-", the string is padded to 2 characters. If the delimiter is "." the string is padded to 4 characters:

0:0:c:7:ac:0 -> 00:00:0c:07:ac:00 0-0-c-7-ac-0 -> 00-00-0c-07-ac-00 0.c07.ac00 -> 0000.0c07.ac00

If the string consists of anything other than hex chars and a single delimiter the string is returned unmodified.