feat(server): instance "name", store pipes in stdpath(state)

Problem:
- Unix sockets are created in random /tmp dirs.
  - /tmp is messy, unclear when OSes actually clear it.
  - The generated paths are very ugly. This adds friction to reasoning
    about which paths belong to which Nvim instances.
- No way to provide a human-friendly way to identify Nvim instances in
  logs or server addresses.

Solution:
- Store unix sockets in stdpath('state')
- Allow --listen "name" and serverstart("name") to given a name (which
  is appended to a generated path).

TODO:
- is stdpath(state) the right place?

src/nvim/eval/funcs.c

@@ -8497,7 +8497,7 @@ static void f_serverstart(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       address = xstrdup(tv_get_string(argvars));
     }
   } else {
-    address = server_address_new();
+    address = server_address_new(NULL);
   }
 
   int result = server_start(address);