When viewing attachments externally with a (non-copiousoutput) mailcap
entry missing %s, the command is invoked as a filter, with the
attachment piped into stdin.  However, unlike a filter, the user
interacts with the command, instead of just displaying the output in
the pager.

Just as with the mutt_system() command, Mutt needs to send imap
keepalives to keep those connections from closing during the
potentially extended invocation.

Thanks to John Hawkinson for the bug report, and his suggested patch,
which this commit is based upon.