doctor: add package for running in-depth healthchecks; use in bugreport (#5413)

Change-Id: Iaa4e5b021a545447f319cfe8b3da2bd3e5e5782b
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

types/logger/logger.go

@@ -138,6 +138,8 @@ var rateFree = []string{
 	"SetPrefs: %v",
 	"peer keys: %s",
 	"v%v peers: %v",
+	// debug messages printed by 'tailscale bugreport'
+	"diag: ",
 }
 
 // RateLimitedFn is a wrapper for RateLimitedFnWithClock that includes the