BSD: implements the 'if_eui64' function in configure.c

(imported from babeld/kernel_socket.c)
ahcp-config.sh: implements ahcp-config.sh for MacOS X.

Tested with MacOS X 10.7 (Lion).
ahcp-config.sh: circumvent a bug in MacOS X 10.6 (Snow Leopard).

On a 64bit processor with MacOS X 10.6, and probably also with
previous releases, the 'networksetup' command improperly parses IPv6
addresses. Use the low-level 'scutil' command to manually fix the IP.