pgsql: Add functions to reset the statistics counter for a single

Log Message:
-----------
Add functions to reset the statistics counter for a single table/index or
a single function.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        monitoring.sgml (r1.74 -> r1.75)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.74&r2=1.75)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.198 -> r1.199)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.198&r2=1.199)
    pgsql/src/backend/utils/adt:
        pgstatfuncs.c (r1.58 -> r1.59)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c?r1=1.58&r2=1.59)
    pgsql/src/include:
        pgstat.h (r1.87 -> r1.88)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h?r1=1.87&r2=1.88)
    pgsql/src/include/catalog:
        catversion.h (r1.579 -> r1.580)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.579&r2=1.580)
        pg_proc.h (r1.565 -> r1.566)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.565&r2=1.566)