Make mg_connect_{http,ws} use mg_parse_uri

Remove the specialized URI parser, clean up code a bit.

Fix parsing of URIs with IPv6 hosts (http://[2001:2:3::4]:567/)

PUBLISHED_FROM=968ad97585d928123106ce3828920ee073113f83

docs/c-api/http.h/intro.md

@@ -11,6 +11,7 @@ items:
   - { name: mg_send_websocket_handshake.md }
   - { name: mg_send_websocket_handshake2.md }
   - { name: mg_send_websocket_handshake3.md }
+  - { name: mg_send_websocket_handshake3v.md }
   - { name: mg_set_protocol_http_websocket.md }
   - { name: mg_url_decode.md }
   - { name: struct_http_message.md }