xcpdb|compact: support some xapian-compact switches
Allow users to specify the --blocksize <B>, --no-full, --fuller
options for xapian-compact(1) for fine-tuning compact behavior
for low-traffic/inactive inboxes.
We also won't support --multipass, since it doesn't seem
compatible with our requirement to use --no-renumber.
We also won't support --single-file, since it only seems
intended for totally dead inboxes; and it doesn't seem
worth the support overhead when "totally dead" turns out
to be a misdiagnosis.