tramp: recovering from a dead connection

[Yuri D'Elia <wavexx@thregr.org>]

Using tramp on the current master has some issues when it comes down to
flaky links.

I was editing a couple of buffers through the scp transport. Link died,
so the ssh process terminated. I just wanted to kill the buffers.
Everything I do seems to fail..

Tramp: Opening connection nil for remote_host using scp...failed
File error: Couldn't find command to check if file exists
tramp-error: Couldn't find command to check if file exists
Tramp: Opening connection nil for remote_host using scp...failed
Tramp: Opening connection nil for remote_host using scp...failed
Tramp: Opening connection nil for remote_host using scp...failed
Tramp: Opening connection nil for remote_host using scp...failed
Tramp: ‘getconf PATH’ not successful, using default value "/bin:/usr/bin".
Tramp: Opening connection nil for remote_host using scp...failed
File error: Couldn't find command to check if file exists
tramp-error: Couldn't find command to check if file exists

I tried first tramp-cleanup-this-connection, then all-connections,
however all I got is to get even more stuck. M-x doesn't even bring the
minibuffer prompt anymore when run within the stuck buffer.

I could use C-x C-b to get into a buffer list, however that also
only contains a single line: "ssh: could not connect to ...".
Fortunately, from there M-x works. But I still cannot terminate those
buffers.