pgsql: Tweak the behavior of log_duration as proposed by Guillaume Smet:

Log Message:
-----------
Tweak the behavior of log_duration as proposed by Guillaume Smet: rather
than being equivalent to setting log_min_duration_statement to zero, this
option now forces logging of all query durations, but doesn't force logging
of query text.  Also, add duration logging coverage for fastpath function
calls.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.84&r2=1.85)
    pgsql/src/backend/tcop:
        fastpath.c (r1.91 -> r1.92)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/fastpath.c.diff?r1=1.91&r2=1.92)
        postgres.c (r1.507 -> r1.508)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.507&r2=1.508)
    pgsql/src/include/tcop:
        tcopprot.h (r1.82 -> r1.83)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/tcopprot.h.diff?r1=1.82&r2=1.83)