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.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        protocol.sgml (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/protocol.sgml?r1=1.70&r2=1.71)
    pgsql/src/interfaces/libpq:
        fe-protocol3.c (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-protocol3.c?r1=1.31&r2=1.32)