resolved: create private stub resolve file for /etc/resolv.conf integration (#7014)

resolved: create private stub resolve file for /etc/resolv.conf integration (#7014)

This creates a second private resolve.conf file which lists the stub resolver
and the resolved acquired search domains.

This runtime file should be used as a symlink target for /etc/resolv.conf such
that non-nss based applications can resolve search domains.

Fixes: #7009