Make socket location configurable

On vagrant using the rails root as the socket location is quite a hassle
since by default synced folders are shared so that creating a socket
inside is not possible. By making the location configurable via ZEUSSOCK
environment variable, such usage get way easier.

I moved the logic for the location to unixsocket to have it in a
location already referenced by both using modules, and not duplicating
it as it was before by setting the const string.
menomize sockName

so we don't need to get it more than once from the env
init sockName to some default value to check against

get rid of the bool to check if we set yet
init() expresses this thought a bit more cleanly.
Merge branch 'configurable_sock_location'