put: degrade to the anonymous FTP queue when the SSH transport fails

pkh put only spoke SFTP to the PPA queue, so a failure of the SSH
transport itself (TCP, banner exchange) failed the upload even though
dput happily pushes the same files: its plain ppa: profile goes over
the anonymous FTP queue of ppa.launchpad.net, the same destination
over another port.

Classify the SSH connection failures: Transport (the connection never
came up: resolution, TCP, banner or key exchange) degrades to that FTP
queue — the upload order (payload first, .changes last), the
reverse-order DELE cleanup of a failed upload and the per-chunk
progress reporting all mirror the SFTP path, sharing cleanup_list.
Refused failures (host key not accepted, no matching authentication)
stay errors: silently switching transport would bypass the refusal.

The FTP client is suppaftp's blocking stream, with the time bounds it
does not carry by itself: the control channel's reads and writes, the
data channel's writes and connect (through a custom passive stream
builder), and the NAT workaround for PASV replies announcing an
unroutable address. The queue endpoints (host, port) join
data/launchpad.yml next to the SFTP ones, and the FTP transport is
covered by unit tests against an in-process fake queue plus a live
control-channel handshake with the real server (ignored, network).

README.md

@@ -133,6 +133,7 @@ Missing features:
   - [ ] Show build status
 - [ ] `pkh put`
   - [x] Upload the source package to a PPA (native SFTP, no `dput` dependency)
+  - [x] Degrade to the anonymous FTP queue when the SSH connection never comes up (dput's upload method)
   - [ ] Upload the source package to the archive
 - [ ] `pkh commit`
   - [ ] Commit the changes to git