This change is to restore /etc/resolv.conf after tailscale down is called. This is done by setting the dns.Manager before errors occur. Error collection is also added.
Fixes#723
* wgengine/router/router_linux.go: Switched `cidrDiff("addr")` and `cidrDiff("route")` order
Signed-off-by: Christina Wen <christina@tailscale.com>
Co-authored-by: Christina Wen <christina@tailscale.com>