pgsql: Tweak TCP-keepalive code so that an invalid setting doesn't cause

Log Message:
-----------
Tweak TCP-keepalive code so that an invalid setting doesn't cause us
to drop connections unceremoniously.  Also some other marginal cleanups:
don't query getsockopt() repeatedly if it fails, and avoid having the
apparent definition of struct Port depend on which system headers you
might have included or not.  Oliver Jowett and Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        pqcomm.c (r1.178 -> r1.179)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c.diff?r1=1.178&r2=1.179)
    pgsql/src/backend/utils/misc:
        guc.c (r1.287 -> r1.288)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.287&r2=1.288)
        postgresql.conf.sample (r1.165 -> r1.166)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.165&r2=1.166)
    pgsql/src/include/libpq:
        libpq-be.h (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h.diff?r1=1.50&r2=1.51)