Learn "--server location" command line option.

Move server initialization from os/event.c to init_server() in main.c.
Use $NVIM_LISTEN_ADDRESS only if "--server" is not specified.
path: Return bool from add_pathsep().
server: Ignore NVIM_LISTEN, make server list.

Create a list of servers in ~/.nvim/servers/{pid} so that every server
instance can be discoverable.

Ignore NVIM_LISTEN_ADDRESS to prevent 'addr in use' errors.