Like btl_tcp_disable_family, this parameter more or less disables
a whole address family. Though the sockets are still created, the
corresponding information isn't added to the connection strings.
Likewise, we don't try to connect to addresses matching the disabled
address family.
This is particularly important for multidomain clusters, where IPv4 is
oftenly filtered (firewalled), sometimes by simply dropping the packets
instead of rejecting them (thus causing a connection timeout instead of
a quick "no route to host").
This commit was SVN r18163.
0ddfff4ffe