cmd/tailscale,net/netcheck: add debug feature to force preferred DERP

This provides an interface for a user to force a preferred DERP outcome
for all future netchecks that will take precedence unless the forced
region is unreachable.

The option does not persist and will be lost when the daemon restarts.

Updates tailscale/corp#18997
Updates tailscale/corp#24755

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -3013,6 +3013,14 @@ func (c *Conn) DebugPickNewDERP() error {
 	return errors.New("too few regions")
 }
 
+func (c *Conn) DebugForcePreferDERP(n int) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.logf("magicsock: [debug] force preferred DERP set to: %d", n)
+	c.netChecker.SetForcePreferredDERP(n)
+}
+
 // portableTrySetSocketBuffer sets SO_SNDBUF and SO_RECVBUF on pconn to socketBufferSize,
 // logging an error if it occurs.
 func portableTrySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {