allow host name to start with forward slash

postgresql allows a hostname whose first char is a #\/ to mean that we should connect to a unix socket in that directory; this patch allows cl-postgres to accept the same.

while this functionality was already possible via a host of `:unix` and setf'ing `*unix-socket-dir*`, this makes the code slightly easier to use in the case where the host and post come from an external configuration file.

(sorry if this is confusing, but i have two things i would like to submit so i had to recreate this pull request on a seperate branch)