Maybe set up some preferences for this? Default subject/message text?

The exim sql driver defines a lot of stuff in conf.php that should
already be defined in Horde/IMP's configs. We should probably reuse
the Horde/IMP stuff rather than duplicating the configuration?

New database files should be configurable per realm, rather than as a
global settings, since each realm could (though unlikely) use a
different vacation file format.

Alias support should be a config option, as not all vacation programs
support the -a switch (but most do). Sufficient to add/remove it from
the form based on a config item.

Add support for multiple aliases rather than just one.

Add support for vacation -t and/or -r switch. vacation on FreeBSD
accepts -t for compatibility, but ignores it. vacation on
NetBSD/OpenBSD doesn't have a -t option. These three suport -r, but
the granularity is days rather than seconds/minutes/hours/days/weeks.
