nss: always explicitly reset all error variables

glibc appears to be broken if we don't explicitly reset all error
variables, let's work around that.

https://bugzilla.redhat.com/show_bug.cgi?id=1125975
resolved: beef up DNS server configuration logic

We now maintain two lists of DNS servers: system servers and fallback
servers.

system servers are used in combination with any per-link servers.

fallback servers are only used if there are no system servers or
per-link servers configured.

The system server list is supposed to be populated from a foreign tool's
/etc/resolv.conf (not implemented yet).

Also adds a configuration switch for LLMNR, that allows configuring
whether LLMNR shall be used simply for resolving or also for responding.