FreeBSD port (55): getifaddrs, iteration through list of interfaces

[Roland McGrath]

> The SIOCGIFCONF ioctl on FreeBSD differs from the one on Linux in two ways:
> - The list it returns consists of entries of varying length.
> - Not all entries refer to enabled interfaces.

Again, this change needs comments in the code itself explaining its
nonobvious actions.  Anyway, isn't it better to check the flags for IFF_UP
or something rather than just ignore a failing ioctl as the sign?

You also changed the behavior in a way not mentioned in your remarks or
your log entry.  That is, ignoring a failing SIOCGIFFLAGS rather than bailing.

I have put in the __if_nextreq changes, but not the others.