[bugfix] Fix the issue of incorrect mac address - #68

AddressToMacResolver formatted each octet with {:X}, which drops the
leading zero: a client at 01:AF:BC:00:0A:FF was reported as 1:AF:BC:0:A:FF.
MacAuthorizer does an exact lookup against the configured entries, which
IP.IsAcceptedMAC requires to be six two-digit groups, so any client whose
MAC contains an octet below 0x10 could never be admitted -- roughly a third
of all addresses.

This is the same one-character fix proposed upstream in nmaier#68
by @cyclamenkde; credit for spotting it is theirs. It is included here
because the configuration file added in the following commits makes MAC
restrictions a first-class, per-server setting, and shipping that on top of
a known matching bug would be misleading.

Note that the MAC lookup itself (iphlpapi!SendARP) only exists on Windows.
Elsewhere no MAC is resolved and MAC restrictions never match; that
limitation predates the port and is documented rather than changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>