safely handle failure to open hosts, services, resolv.conf files

previously, transient failures like fd exhaustion or other
resource-related errors were treated the same as non-existence of
these files, leading to fallbacks or false-negative results. in
particular:

- failure to open hosts resulted in fallback to dns, possibly yielding
  EAI_NONAME for a hostname that should be defined locally, or an
  unwanted result from dns that the hosts file was intended to
  replace.

- failure to open services resulted in EAI_SERVICE.

- failure to open resolv.conf resulted in querying localhost rather
  than the configured nameservers.

now, only permanent errors trigger the fallback behaviors above; all
other errors are reportable to the caller as EAI_SYSTEM.

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