add: support running admin server on unix socket

This is useful when multiple instances run on the same machine, for
example behind a proxy. Unix sockets for web and admin servers can then
be put in the same folder for each instance.

Can be helpful when writing tests as well.

test/io/configs/expected/jspath-str-op-dump5.config

@@ -1,5 +1,7 @@
 admin-server-host = "!4"
 admin-server-port = ""
+admin-server-unix-socket = ""
+admin-server-unix-socket-mode = "660"
 client-error-verbosity = "verbose"
 db-aggregates-enabled = false
 db-anon-role = ""