make unix socket path configurable

Hello,
we have the requirement here to serve at least 2 communities on one gateway. Hence, map A has to ask alfred A for data and map B has to ask alfred B for data.

This is also upstream in alfred daemon: http://git.open-mesh.org/alfred.git/commit/6c6a6f735054a839bd809d5711c29287505ccdd5

This patch should be backwards compatible because ALFRED_SOCK_PATH_DEFAULT points still to "/var/run/alfred.sock".

Would be great if someone could also patch the ffmap-backend scripts as well...