pgsql: Add XMLText function (SQL/XML X038)

Add XMLText function (SQL/XML X038)

This function implements the standard XMLTest function, which
converts text into xml text nodes. It uses the libxml2 function
xmlEncodeSpecialChars to escape predefined entities (&"<>), so
that those do not cause any conflict when concatenating the text
node output with existing xml documents.

This also adds a note in  features.sgml about not supporting
XML(SEQUENCE). The SQL specification defines a RETURNING clause
to a set of XML functions, where RETURNING CONTENT or RETURNING
SEQUENCE can be defined. Since PostgreSQL doesn't support
XML(SEQUENCE) all of these functions operate with an
implicit RETURNING CONTENT.

Author: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://postgr.es/m/86617a66-ec95-581f-8d54-08059cca8885@uni-muenster.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/526fe0d79914b2dfcfd79effd1ab26ff62469248

Modified Files
--------------
doc/src/sgml/features.sgml           |  9 +++++++++
doc/src/sgml/func.sgml               | 30 ++++++++++++++++++++++++++++++
src/backend/catalog/sql_features.txt |  2 +-
src/backend/utils/adt/xml.c          | 22 ++++++++++++++++++++++
src/include/catalog/pg_proc.dat      |  3 +++
src/test/regress/expected/xml.out    | 36 ++++++++++++++++++++++++++++++++++++
src/test/regress/expected/xml_1.out  | 23 +++++++++++++++++++++++
src/test/regress/expected/xml_2.out  | 36 ++++++++++++++++++++++++++++++++++++
src/test/regress/sql/xml.sql         |  7 +++++++
9 files changed, 167 insertions(+), 1 deletion(-)