cql2pqf: refactor truncation handling and fix Z39.58 mode

cql2pqf: refactor truncation handling and fix Z39.58 mode

Refactor the CQL to PQF term code in cqltransform.c. Characters that
don't need escaping aren't and characters that do need escaping are.
For example CQL query a#a? must be handled as a Z39.58 mode query
because the trailing ? (single-character mask in CQL). But since the
generated term includes #, it must be escaped and survive the PQF parse,
thus the result is "\\#a#" (was "a#a# before). Also the CQL term a*3 now
produces "a?\\3" rather than "a?3" . This is becauase a?n has special
meaning in Z39.58. Characters that do need escaping are the PQF specials
" and \\. Rest are preseved and \-sequence in CQL are otherwise removed.