Create the socket in HOME

Support VOLTRON_SOCKET for running more than one voltron

Makes running more than one instance at a time feasible (it was doable before by constantly moving the socket about in /tmp and maintaining an elaborate web of symlinks). Now just set VOLTRON_SOCKET to something unique before you start tmux.

For bonus points this should create `~/.voltron/voltron.sock.$inferior_pid` when you start an inferior, but I couldn't find the magic to do it in the 4 minutes I looked.