accounting: bwlimit signal handler should always start

The SIGUSR2 signal handler for bandwidth limits currently only starts
if rclone is started at a time when a bandwidth limit applies. This
means that if rclone starts _outside_ such a time, i.e. with no
bandwidth limits, then enters a time where bandwidth limits do apply,
it will not be possible to use SIGUSR2 to toggle it.

This fixes that by always starting the signal handler, but only
toggling the limiter if there is a bandwidth limit configured.

fs/accounting/accounting_unix.go

@@ -23,15 +23,26 @@ func (tb *tokenBucket) startSignalHandler() {
 		// This runs forever, but blocks until the signal is received.
 		for {
 			<-signals
-			tb.mu.Lock()
-			tb.toggledOff = !tb.toggledOff
-			tb.curr, tb.prev = tb.prev, tb.curr
-			s := "disabled"
-			if !tb.curr._isOff() {
-				s = "enabled"
-			}
-			tb.mu.Unlock()
-			fs.Logf(nil, "Bandwidth limit %s by user", s)
+
+			func() {
+				tb.mu.Lock()
+				defer tb.mu.Unlock()
+
+				// if there's no bandwidth limit configured now, do nothing
+				if !tb.currLimit.Bandwidth.IsSet() {
+					fs.Debugf(nil, "SIGUSR2 received but no bandwidth limit configured right now, ignoring")
+					return
+				}
+
+				tb.toggledOff = !tb.toggledOff
+				tb.curr, tb.prev = tb.prev, tb.curr
+				s := "disabled"
+				if !tb.curr._isOff() {
+					s = "enabled"
+				}
+
+				fs.Logf(nil, "Bandwidth limit %s by user", s)
+			}()
 		}
 	}()
 }