Fix non-windows/non-unix builds for bwlimit/SIGUSR2 feature and add a mutex

The race detector complained whenever SIGUSR2 was sent to rclone so
this adds a mutex to prevent concurrent access.

fs/accounting_other.go

@@ -0,0 +1,10 @@
+// Accounting and limiting reader
+// Non-unix specific functions.
+
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
+
+package fs
+
+// startSignalHandler() is Unix specific and does nothing under non-Unix
+// platforms.
+func startSignalHandler() {}