isearch: use numeric sort for article numbers

isearch: use numeric sort for article numbers

Perl sort is alphabetical by default and Xapian uses numeric
document IDs, so sort must be told explicitly to use numeric
comparisons even if the scalars are integer values (IV)
internally.

And eliminate extra hash marks ("#") since they're probably too
noisy if there are many IDs.

Note: I haven't seen this warning message in syslog, yet :>