[9fans] drawterm

[Russ Cox <rsc@swtch.com>]

I made some changes to drawterm yesterday that fix some
of the snarf problems and might even fix the random crashes
that some have seen (and others have not).  I also replaced
my pipe-based pseudo-locks with real pthreads locks, so if it
seemed slow before, maybe it will be better now.

Best of all, I have a test case that reliably crashes drawterm.
If you connect to somewhere and then run
    fcp /mnt/term/some/big/file /dev/null
then eventually drawterm or the remote kernel or both get
confused about what's going on on the wire and someone
hangs up.  It never gets through more than a couple megabytes
before this happens.

Drawterm gets a read error from "bad digest", and the
kernel sometimes sees unexpected rpc messages, usually
Rreads to Treads that it is trying to flush (but the Rflush is
sent only after the Rread, so the Rread is legitimate).

I'm out of ideas at the moment.  If someone else wants to debug,
please do so.

   cvs -d :pserver:anonymous@cvs.pdos.csail.mit.edu:/cvs co drawterm

Russ