Fall back to ipv4 if dual stack sockets are disabled

[rustybee42]

Implement the same checks as in beegfs-core to have similar behavior: If dual stack is disabled (or ipv6 is enabled during boot but disabled at runtime), fall back to ipv4.

[iamjoemccormick]

Just a couple of small suggestions, but I'll leave it up to you if these things are important to resolve or if the code is robust enough as is.

[iamjoemccormick]

Meant to hit "approve" instead of "request changes" because I don't consider my feedback blocking.