[Evolution] libibex use-limiting patch applied

[Not Zed <notzed ximian com>]

Hi all (esp miguel),

I've put changes into libibex that limit the number of open ibex files
it tries to maintain at any given instance, in order to reduce the
number of file descriptors needed for many open folders.

I can't say i've tested it heavily, but it seems to work as i expect,
and limit the number of open ibex files to 5 at a maximum
(ibex_internal.h:IBEX_OPEN_THREASHOLD), closing the least-recently used
files as new ones are required.

One main concern i have is tuning the cache size so that operations
aren't too slow.  e.g. if you have a body-search based vfolder over 20
folders, it will probably thrash a lot when things get updated.
(but then opening/querying/closing ibex files shouldn't be very very
slow).

 Z