derp: track client-advertised non-ideal DERP connections in more places

In f77821fd63 (released in v1.72.0), we made the client tell a DERP server
when the connection was not its ideal choice (the first node in its region).

But we didn't do anything with that information until now. This adds a
metric about how many such connections are on a given derper, and also
adds a bit to the PeerPresentFlags bitmask so watchers can identify
(and rebalance) them.

Updates tailscale/corp#372

Change-Id: Ief8af448750aa6d598e5939a57c062f4e55962be
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/depaware.txt

@@ -155,7 +155,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/clientmetric                              from tailscale.com/net/netcheck+
         tailscale.com/util/cloudenv                                  from tailscale.com/net/dnscache+
         tailscale.com/util/cmpver                                    from tailscale.com/net/tshttpproxy+
-        tailscale.com/util/ctxkey                                    from tailscale.com/types/logger
+        tailscale.com/util/ctxkey                                    from tailscale.com/types/logger+
      💣 tailscale.com/util/deephash                                  from tailscale.com/util/syspolicy/setting
    L 💣 tailscale.com/util/dirwalk                                   from tailscale.com/metrics
         tailscale.com/util/dnsname                                   from tailscale.com/cmd/tailscale/cli+