Rewrite Bonjour SOCKS5 communication using Gio.

* Remove XepXfer->rx_len, which is always 0.
* Rewrite Bonjour SOCKS5 communication using Gio.
The `bonjour_sock5_request_cb` function is confusing, incrementing `xf->sock5_req_state`, and then calling itself again to get to the next part of the switch. Using individual callback functions is required by Gio, but also hopefully clearer.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/215/