mtglib/dcprobe: unauthenticated DC verification probe
New leaf package that performs the first step of the MTProto handshake
(req_pq_multi -> resPQ) over the existing obfuscated2 transport. No
auth_key is generated; no long-lived state is introduced. Two TL
messages, one round-trip, no new dependencies.
A generic listener cannot fake the reply because it must echo back our
random nonce in resPQ.
Used by the doctor command in a follow-up commit to distinguish a real
Telegram DC from a generic TCP listener bound to port 443.
Alexey Dolotov