util-lib: add ether_addr_from_string()

util-lib: add ether_addr_from_string()

This isn't quite symmetrical to in_addr_from_string() because it also returns
an offset indicating how much of the string was consumed by the matched
pattern.  This offset reporting is needed for either of the following use
cases:

 * verifying the lack of trailing garbage after such an address
 * parsing subsequent data from the same string