Added Tunnel directive to allow the user to specify a shell command to run
to set up an IMAP connection in place of a TCP socket (eg., to run over
an SSH session).
sync_mailbox() didn't consider MaxSize == 0 to mean "unlimited".
load_config() needs to print a newline in its error messages since
next_arg() kills the newline of the line that was read out of the config
file.
