server: Allow multiple addresses in $NVIM_LISTEN_ADDRESS.

Minor annoyance: When I start multiple instances of nvim (like in compile/test/edit cycles) and every time I start it I read "Failed to start server...Pres ENTER...".

Solution: Let `$NVIM_LISTEN_ADRRESS` contain more than one choice and if they're all being used, just make another and add it to the env variable.

With this, a client could theoretically be written to synchronize information between two or more `nvim`s by attempting to connect to each instance, reading data from one and writing to the other. Though I really just wanted to get rid of that nuisance, it could be more generally useful.