pgsql: Rename the newly-added commands for discarding session state.

Log Message:
-----------
Rename the newly-added commands for discarding session state.
RESET SESSION, RESET PLANS, and RESET TEMP are now DISCARD ALL,
DISCARD PLANS, and DISCARD TEMP, respectively. This is to avoid
confusion with the pre-existing RESET variants: the DISCARD
commands are not actually similar to RESET. Patch from Marko
Kreen, with some minor editorialization.

Modified Files:
--------------
pgsql/doc/src/sgml:
reference.sgml (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/reference.sgml.diff?r1=1.62&r2=1.63)
pgsql/doc/src/sgml/ref:
allfiles.sgml (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/allfiles.sgml.diff?r1=1.69&r2=1.70)
reset.sgml (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/reset.sgml.diff?r1=1.34&r2=1.35)
pgsql/src/backend/commands:
Makefile (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/Makefile.diff?r1=1.35&r2=1.36)
pgsql/src/backend/nodes:
copyfuncs.c (r1.373 -> r1.374)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.373&r2=1.374)
equalfuncs.c (r1.304 -> r1.305)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.304&r2=1.305)
pgsql/src/backend/parser:
gram.y (r2.589 -> r2.590)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.589&r2=2.590)
keywords.c (r1.186 -> r1.187)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/keywords.c.diff?r1=1.186&r2=1.187)
pgsql/src/backend/tcop:
utility.c (r1.277 -> r1.278)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.277&r2=1.278)
pgsql/src/backend/utils/misc:
guc.c (r1.388 -> r1.389)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.388&r2=1.389)
pgsql/src/backend/utils/mmgr:
portalmem.c (r1.101 -> r1.102)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/portalmem.c.diff?r1=1.101&r2=1.102)
pgsql/src/bin/psql:
tab-complete.c (r1.161 -> r1.162)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c.diff?r1=1.161&r2=1.162)
pgsql/src/include/nodes:
nodes.h (r1.198 -> r1.199)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/nodes.h.diff?r1=1.198&r2=1.199)
parsenodes.h (r1.346 -> r1.347)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.346&r2=1.347)
pgsql/src/test/regress/expected:
guc.out (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/guc.out.diff?r1=1.3&r2=1.4)
pgsql/src/test/regress/sql:
guc.sql (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/guc.sql.diff?r1=1.3&r2=1.4)

Added Files:
-----------
pgsql/doc/src/sgml/ref:
discard.sgml (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/discard.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/src/backend/commands:
discard.c (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/discard.c?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/src/include/commands:
discard.h (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/discard.h?rev=1.1&content-type=text/x-cvsweb-markup)