uri: fix incorrect parse of unix socket URI with query

Previously in case when unix socket URI with query didn't
contain "unix/:" prefix, parsing of this URI was incorrect.
For example "/unix.sock?q=v" was parsed into "path: /unix.sock"
and "query: x=y" instead of "host: unix/", "service: /unix.sock",
"unix: /unix.sock" and "query: x=y". Now parsing of unix socket
URI is independent from "unix/:" prefix and correct.
Pay attention, all parsing logic is realized in uri.rl file which
compiled in uri.c using 'ragel'.

Part of #5928