pgsql: Fix an ancient oversight in libpq's handling of V3-protocol COPY

Log Message:
-----------
Fix an ancient oversight in libpq's handling of V3-protocol COPY OUT mode:
we need to be able to swallow NOTICE messages, and potentially also
ParameterStatus messages (although the latter would be a bit weird),
without exiting COPY OUT state. Fix it, and adjust the protocol documentation
to emphasize the need for this. Per off-list report from Alexander Galler.

Tags:
----
REL7_4_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
protocol.sgml (r1.47 -> r1.47.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/protocol.sgml?r1=1.47&r2=1.47.2.1)
pgsql/src/interfaces/libpq:
fe-protocol3.c (r1.9.2.1 -> r1.9.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-protocol3.c?r1=1.9.2.1&r2=1.9.2.2)