cmount: don't call host.Umount if a signal has been received

Before this change cgofuse and libatexit would race to see who could
unmount the file system with unpredicatable results. On Linux it could
report an error or not, depending.

This change checks to see if umount is beng called from a signal and
if so leaves the unmounting to cgofuse/libfuse.

See #4804

cmd/cmount/mount.go

@@ -19,6 +19,7 @@ import (
 	"github.com/pkg/errors"
 	"github.com/rclone/rclone/cmd/mountlib"
 	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/lib/atexit"
 	"github.com/rclone/rclone/vfs"
 )
 
@@ -187,9 +188,17 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (<-chan error
 	unmount := func() error {
 		// Shutdown the VFS
 		fsys.VFS.Shutdown()
-		fs.Debugf(nil, "Calling host.Unmount")
-		if host.Unmount() {
-			fs.Debugf(nil, "host.Unmount succeeded")
+		var umountOK bool
+		if atexit.Signalled() {
+			// If we have received a signal then FUSE will be shutting down already
+			fs.Debugf(nil, "Not calling host.Unmount as signal received")
+			umountOK = true
+		} else {
+			fs.Debugf(nil, "Calling host.Unmount")
+			umountOK = host.Unmount()
+		}
+		if umountOK {
+			fs.Debugf(nil, "Unmounted successfully")
 			if runtime.GOOS == "windows" {
 				if !waitFor(func() bool {
 					_, err := os.Stat(mountpoint)