If systemd-resolved is enabled but not running (or not yet running,
such as early boot) and resolv.conf is old/dangling, we weren't
detecting systemd-resolved.
This moves its ping earlier, which will trigger it to start up and
write its file.
Updates #3362 (likely fixes)
Updates #3531 (likely fixes)
Change-Id: I6392944ac59f600571c43b8f7a677df224f2beed
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>