Utility for one-time down/upgrade of Exim message-id formats
in spool files.  Only the filenames and first-line ID tag values
are affected; not message content such as Message-ID fields.
Only -H, -D and -J files are handled.

Usage: exim_id_update [-d | -u | -h | -v] [spooldir]

	-d --downgrade downgrade mode
	-h --help      help message
	-u --upgrade   upgrade mode
	-v --version   show version and exit cleanly
	--verbose      more output about what's going on
	--force        force overwriting (may be required after failure)
	--dry          dry run (do file operations, but cleanup and keep the old files)

Exactly one of -d|--downgrade or -u|--upgrade must be given.
The spool directory defaults to the build-time value,
or can be given as a command-line argument.