pgsql: Improve the recently-added libpq events code to provide more

Log Message:
-----------
Improve the recently-added libpq events code to provide more consistent
guarantees about whether event procedures will receive DESTROY events.
They no longer need to defend themselves against getting a DESTROY
without a successful prior CREATE.

Andrew Chernow

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.261 -> r1.262)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.261&r2=1.262)
    pgsql/src/interfaces/libpq:
        fe-exec.c (r1.198 -> r1.199)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-exec.c?r1=1.198&r2=1.199)
        libpq-events.c (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-events.c?r1=1.1&r2=1.2)
        libpq-int.h (r1.132 -> r1.133)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h?r1=1.132&r2=1.133)