pgsql: Arrange for the authentication request type to be preserved in

Log Message:
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Arrange for the authentication request type to be preserved in
PGconn. Invent a new libpq connection-status function,
PQconnectionUsedPassword() that returns true if the server
demanded a password during authentication, false otherwise.
This may be useful to clients in general, but is immediately
useful to help plug a privilege escalation path in dblink.
Per list discussion and design proposed by Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.235 -> r1.236)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.235&r2=1.236)
    pgsql/src/include/libpq:
        pqcomm.h (r1.102 -> r1.103)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/pqcomm.h.diff?r1=1.102&r2=1.103)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt.diff?r1=1.15&r2=1.16)
        fe-connect.c (r1.345 -> r1.346)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c.diff?r1=1.345&r2=1.346)
        libpq-fe.h (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-fe.h.diff?r1=1.136&r2=1.137)
        libpq-int.h (r1.119 -> r1.120)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h.diff?r1=1.119&r2=1.120)