When printing MAC addresses, always use trailing zeroes.

When printing MAC addresses, always use trailing zeroes.

tinc currently prints MAC addresses without trailing zeroes, for example:

    1:2:3:4:5:6

This looks weird and is inconsistent with how MAC addresses are
displayed everywhere else. This commit adds trailing zeroes, so the
above address will be printed as the following:

    01:02:03:04:05:06