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).

data/launchpad.yml

@@ -13,6 +13,9 @@
 ##   ssh_*:        the PPA upload queue, as expanded by dput-ng's
 ##                 ppa:user/ppa profile (ppa.launchpad.net:22, incoming
 ##                 ~<user>/<ppa>)
+##   ftp_*:        the same upload queue over anonymous FTP, dput-ng's
+##                 plain ppa: profile: the transport pkh degrades to
+##                 when the SSH connection itself never comes up
 ##   content_host_template: ppa.launchpadcontent.net serves PPA apt
 ##                 repositories since the 2022 move off ppa.launchpad.net
 ##   git_web_template: Launchpad's CGit mirrors of Ubuntu source packages
@@ -21,6 +24,10 @@
 api_base: https://api.launchpad.net/1.0
 ssh_host: ppa.launchpad.net
 ssh_port: 22
+## The anonymous FTP upload queue dput-ng's plain ppa: profile uses:
+## pkh degrades to it when the SSH connection itself never comes up.
+ftp_host: ppa.launchpad.net
+ftp_port: 21
 incoming_template: "~{owner}/{ppa}"
 content_host_template: https://ppa.launchpadcontent.net/{owner}/{ppa}/ubuntu
 git_web_template: https://git.launchpad.net/ubuntu/+source/{package}