new emacs function for date-restricted searches

[Jameson Rollins]

The following patch introduces a new function for restricting search
results to messages within a certain look-back time.  For instance, if
the time specification '3w' is given then the search will be
restricted to messages received within the last three weeks.

The second patch binds this function to the 'S' key.

I have a feeling that Carl won't be into this patch, but I'm throwing
it out there anyway, just in case.  On my slow machine, use of this
function speeds up searches considerably, especially since most of the
time I'm just searching for recent messages anyway.