Enable truncation attribute in Z39.50 queries to be left out entirely

Enable truncation attribute in Z39.50 queries to be left out entirely

Per https://bugs.launchpad.net/evergreen/+bug/702695, some Z39.50
servers hate the truncation attribute (@attr 5=anything) and always
return 0 hits.

This gives you the ability to specify a negative value for the
truncation attribute and thereby prevent the truncation attribute
from being included in the Z39.50 query for a given server.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_1_6_1@19179 dcc99617-32d9-48b4-a31d-7c20da2025e4