pgsql: Add a hook to let loadable modules get control at ProcessUtility

Log Message:
-----------
Add a hook to let loadable modules get control at ProcessUtility execution,
and use it to extend contrib/pg_stat_statements to track utility commands.

Itagaki Takahiro, reviewed by Euler Taveira de Oliveira.

Modified Files:
--------------
    pgsql/contrib/pg_stat_statements:
        pg_stat_statements.c (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_stat_statements/pg_stat_statements.c?r1=1.8&r2=1.9)
    pgsql/doc/src/sgml:
        pgstatstatements.sgml (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgstatstatements.sgml?r1=1.4&r2=1.5)
    pgsql/src/backend/tcop:
        utility.c (r1.323 -> r1.324)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.323&r2=1.324)
    pgsql/src/include/tcop:
        utility.h (r1.37 -> r1.38)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/utility.h?r1=1.37&r2=1.38)