units: add Conflicts=rescue.service to container-getty@.service

The traditional runlevel 1 is "single user mode", and shuts down all but
the main console.  In systemd, rescue.target provides runlevel1.target.
But it did not shut down logins on secondary consoles... if systemd was
running in a container.

I don't think we strictly need to change this.  But when you look at both
container-getty@.service and getty@.service, you see that both have
IgnoreOnIsolate, but only the latter has Conflicts=rescue.service.

This also makes rescue.target in a container consistent with
emergency.target.  In the latter case, the gettys were already stopped,
because they have a Requires dependency on sysinit.target.

units/container-getty@.service.m4.in

@@ -17,6 +17,11 @@ Before=getty.target
 IgnoreOnIsolate=yes
 ConditionPathExists=/dev/pts/%I
 
+# IgnoreOnIsolate is an issue: when someone isolates rescue.target,
+# tradition expects that we shut down all but the main console.
+Conflicts=rescue.service
+Before=rescue.service
+
 [Service]
 # The '-o' option value tells agetty to replace 'login' arguments with an
 # option to preserve environment (-p), followed by '--' for safety, and then