pgsql: Support setting the keepalive idle time on MacOS X.

Log Message:
-----------
Support setting the keepalive idle time on MacOS X.

MacOS X uses TCP_KEEPALIVE rather than TCP_KEEPIDLE for this purpose.

Thanks to Fujii Masao for the review.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.291 -> r1.292)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.291&r2=1.292)
        libpq.sgml (r1.311 -> r1.312)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.311&r2=1.312)
    pgsql/src/backend/libpq:
        pqcomm.c (r1.209 -> r1.210)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.209&r2=1.210)
    pgsql/src/interfaces/libpq:
        fe-connect.c (r1.395 -> r1.396)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c?r1=1.395&r2=1.396)