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

Log Message:
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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:
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REL8_0_STABLE

Modified Files:
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    pgsql/doc/src/sgml:
        protocol.sgml (r1.57.4.1 -> r1.57.4.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/protocol.sgml?r1=1.57.4.1&r2=1.57.4.2)
    pgsql/src/interfaces/libpq:
        fe-protocol3.c (r1.20 -> r1.20.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-protocol3.c?r1=1.20&r2=1.20.4.1)