Alexey Dolotov
408e2c7150
doctor: deepen DC verification with MTProto handshake probe
Closes #494.
After a successful TCP connect, run an unauthenticated req_pq_multi ->
resPQ exchange via mtglib/dcprobe. This rejects generic listeners that
happen to bind 443 but cannot speak MTProto.
Output now shows "(rpc <rtt>)" on success; on failure the wrapped error
distinguishes "tcp connect to ...: ..." from "rpc handshake to ...: ...".
The probe runs by default — an opt-in flag would defeat the purpose,
since the existing TCP-only check is what motivated the issue.