pgsql: Include a pointer to the query's source text in QueryDesc

Log Message:
-----------
Include a pointer to the query's source text in QueryDesc structs. This is
practically free given prior 8.4 changes in plancache and portal management,
and it makes it a lot easier for ExecutorStart/Run/End hooks to get at the
query text. Extracted from Itagaki Takahiro's pg_stat_statements patch,
with minor editorialization.

Modified Files:
--------------
pgsql/src/backend/commands:
copy.c (r1.303 -> r1.304)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c?r1=1.303&r2=1.304)
explain.c (r1.183 -> r1.184)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.183&r2=1.184)
prepare.c (r1.95 -> r1.96)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c?r1=1.95&r2=1.96)
pgsql/src/backend/executor:
functions.c (r1.131 -> r1.132)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c?r1=1.131&r2=1.132)
spi.c (r1.203 -> r1.204)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.203&r2=1.204)
pgsql/src/backend/tcop:
pquery.c (r1.128 -> r1.129)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c?r1=1.128&r2=1.129)
pgsql/src/include/commands:
explain.h (r1.37 -> r1.38)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/explain.h?r1=1.37&r2=1.38)
pgsql/src/include/executor:
execdesc.h (r1.39 -> r1.40)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/execdesc.h?r1=1.39&r2=1.40)