pgsql: Extend EXPLAIN to support output in XML or JSON format.

Log Message:
-----------
Extend EXPLAIN to support output in XML or JSON format.

There are probably still some adjustments to be made in the details
of the output, but this gets the basic structure in place.

Robert Haas

Modified Files:
--------------
pgsql/contrib/auto_explain:
auto_explain.c (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/auto_explain/auto_explain.c?r1=1.6&r2=1.7)
pgsql/doc/src/sgml:
auto-explain.sgml (r1.3 -> r1.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/auto-explain.sgml?r1=1.3&r2=1.4)
pgsql/doc/src/sgml/ref:
explain.sgml (r1.45 -> r1.46)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/explain.sgml?r1=1.45&r2=1.46)
pgsql/src/backend/commands:
explain.c (r1.188 -> r1.189)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.188&r2=1.189)
prepare.c (r1.98 -> r1.99)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c?r1=1.98&r2=1.99)
pgsql/src/backend/utils/adt:
xml.c (r1.92 -> r1.93)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.92&r2=1.93)
pgsql/src/backend/utils/cache:
lsyscache.c (r1.162 -> r1.163)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c?r1=1.162&r2=1.163)
pgsql/src/backend/utils/sort:
tuplesort.c (r1.92 -> r1.93)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c?r1=1.92&r2=1.93)
pgsql/src/include/commands:
explain.h (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/explain.h?r1=1.40&r2=1.41)
pgsql/src/include/utils:
lsyscache.h (r1.128 -> r1.129)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/lsyscache.h?r1=1.128&r2=1.129)
tuplesort.h (r1.33 -> r1.34)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/tuplesort.h?r1=1.33&r2=1.34)
xml.h (r1.28 -> r1.29)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h?r1=1.28&r2=1.29)