fix getaddrinfo regression with AI_ADDRCONFIG on some configurations

despite not being documented to do so in the standard or Linux
documentation, attempts to udp connect to 127.0.0.1 or ::1 generate
EADDRNOTAVAIL when the loopback device is not configured and there is
no default route for IPv6. this caused getaddrinfo with AI_ADDRCONFIG
to fail with EAI_SYSTEM and EADDRNOTAVAIL on some no-IPv6
configurations, rather than the intended behavior of detecting IPv6 as
unsuppported and producing IPv4-only results.

previously, only EAFNOSUPPORT was treated as unavailability of the
address family being probed. instead, treat all errors related to
inability to get an address or route as conclusive that the family
being probed is unsupported, and only fail with EAI_SYSTEM on other
errors.

further improvements may be desirable, such as reporting EAI_AGAIN
instead of EAI_SYSTEM for errors which are expected to be transient,
but this patch should suffice to fix the serious regression.

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