pgsql: Use new cstring/text conversion functions in some additional

Log Message:
-----------
Use new cstring/text conversion functions in some additional places.
These changes assume that the varchar and xml data types are represented
the same as text. (I did not, however, accept the portions of the proposed
patch that wanted to assume bytea is the same as text --- tgl.)

Brendan Jurd

Modified Files:
--------------
pgsql/contrib/pgcrypto:
pgp-pgsql.c (r1.9 -> r1.10)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/pgp-pgsql.c?r1=1.9&r2=1.10)
pgsql/contrib/xml2:
xpath.c (r1.18 -> r1.19)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.18&r2=1.19)
xslt_proc.c (r1.10 -> r1.11)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.10&r2=1.11)
pgsql/src/backend/utils/adt:
varchar.c (r1.127 -> r1.128)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varchar.c?r1=1.127&r2=1.128)
xml.c (r1.72 -> r1.73)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.72&r2=1.73)