Refactor random string stuff and seeding

Make sure we have a good random seed on the default RNG, that the
predictable RNG is clearly marked as such, that random strings are
actually the length requested, and that they contain a restricted set of
characters only.

cmd/syncthing/gui.go

@@ -353,7 +353,7 @@ func restPostConfig(m *model.Model, w http.ResponseWriter, r *http.Request) {
 		if curAcc := cfg.Options().URAccepted; newCfg.Options.URAccepted > curAcc {
 			// UR was enabled
 			newCfg.Options.URAccepted = usageReportVersion
-			newCfg.Options.URUniqueID = randomString(6)
+			newCfg.Options.URUniqueID = randomString(8)
 			err := sendUsageReport(m)
 			if err != nil {
 				l.Infoln("Usage report:", err)