This can be an infinite loop if the process-status keeps `open'
or `run' even though the connection has actually died.  In
particular, the `imap-wait-for-tag' function can be the cause of
the hanging since it sends the LOGOUT command to the dead server
and waits for the response.  So, I tried modifying it so as to
quit in a certain time.  The patch to imap.el is below.  Anyway,
it needs to be reviewed by someone who is skilled in IMAP.  The
default value of `imap-timeout-seconds' might be too small for
slow connections.