Make unit tests pass on IPv6-only systems, and systems without
localhost addresses (like some FreeBSD jails).
Fixes:
* get_if_addrs_ifaddrs: systems without localhost
* get_if_addrs_ioctl: only works on IPv4 systems
* socket: check IPv4 and IPv6, skip on EPROTONOSUPPORT
* socketpair_ersatz: uses IPv4, skip on EPROTONOSUPPORT
Fixes bug #17632; bugfix on unit tests in 0.2.7.3-rc.
c464a36772 was a partial fix for this issue in #17255;
it was released in unit tests in 0.2.7.4-rc.
Patch by "teor".
