This is called just before closing the connection, and gives every PPL
one last chance to output anything to the user that it might have
buffered.
No functional change: all implementations so far are trivial, except
that the transport layer passes the call on to its higher
layer (because otherwise nothing would do so).
A user reported yesterday that PuTTY can fail to print a userauth
banner message if the server sends one and then immediately slams the
connection shut. The first step to fixing this is making a convenient
way to reproduce that server behaviour.
(Apparently the real use case has to do with account expiry - the
server in question presumably doesn't have enough layer violations to
be able to put the text "Your account has expired" into an
SSH_MSG_DISCONNECT, so instead it does the next best thing and sends
it as a userauth banner immediately before disconnection.)
This allows the 'no trivial auth' option introduced by the previous
commit to be tested. Uppity has grown three new options to make it
accept "none" authentication, keyboard-interactive involving no
prompts, and the perverse sending of USERAUTH_SUCCESS after a
signatureless public-key offer.
The first of those options also enables the analogue in SSH-1; the
other two have no SSH-1 analogues in the first place. (SSH-1 public
key authentication has a challenge-response structure that doesn't
contain any way to terminate the exchange early with success. And the
TIS and CryptoCard methods, which are its closest analogue of k-i,
have a fixed number of prompts, which is not 0.)
This clears up another large pile of clutter at the top level, and in
the process, allows me to rename source files to things that don't all
have that annoying 'ssh' prefix at the top.
