pgsql: Add support for TCP keepalives on Windows, both for backend and

Log Message:
-----------
Add support for TCP keepalives on Windows, both for backend and the new
libpq support.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        pqcomm.c (r1.210 -> r1.211)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.210&r2=1.211)
    pgsql/src/interfaces/libpq:
        fe-connect.c (r1.396 -> r1.397)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c?r1=1.396&r2=1.397)
    pgsql/doc/src/sgml:
        config.sgml (r1.293 -> r1.294)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.293&r2=1.294)
        libpq.sgml (r1.312 -> r1.313)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.312&r2=1.313)