Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/accounting/accounting_unix.go

@@ -0,0 +1,36 @@
+// Accounting and limiting reader
+// Unix specific functions.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package accounting
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/ncw/rclone/fs"
+)
+
+// startSignalHandler() sets a signal handler to catch SIGUSR2 and toggle throttling.
+func startSignalHandler() {
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGUSR2)
+
+	go func() {
+		// This runs forever, but blocks until the signal is received.
+		for {
+			<-signals
+			tokenBucketMu.Lock()
+			bwLimitToggledOff = !bwLimitToggledOff
+			tokenBucket, prevTokenBucket = prevTokenBucket, tokenBucket
+			s := "disabled"
+			if tokenBucket != nil {
+				s = "enabled"
+			}
+			tokenBucketMu.Unlock()
+			fs.Logf(nil, "Bandwidth limit %s by user", s)
+		}
+	}()
+}