Runs tunnel program with stderr closed

This is an old bug, I looked at the code and it still seems to exist (in the sense that STDERR goes to /dev/null).

The original bug is here: https://bugs.debian.org/800621

This is the bug: Given a tunnel set with 'set tunnel=...', mutt appears to run that tunnel program with stderr closed. This causes some programs, such as Dovecot's imap server, to abort with an error when they try to write to stderr. mutt should run the tunnel with stderr pointing somewhere valid; ideally, mutt could capture and display errors if the tunnel fails, but even if mutt wants to discard errors, it should do so by pointing stderr to /dev/null rather than closing it.