pgsql: Add a PQfireResultCreateEvents function to allow applications to

Log Message:
-----------
Add a PQfireResultCreateEvents function to allow applications to mimic the
sequence of operations that libpq goes through while creating a PGresult.
Also, remove ill-considered "const" decoration on parameters passed to
event procedures.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.262 -> r1.263)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.262&r2=1.263)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt?r1=1.20&r2=1.21)
        libpq-events.c (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-events.c?r1=1.2&r2=1.3)
        libpq-events.h (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-events.h?r1=1.1&r2=1.2)