func UnregisterAll() {
	mmu.Lock()
	m = make(map[string]vfs.FileSystem)
	mmu.Unlock()
}
UnregisterAll unregisters all file systems from backend map
package backend
import (
	"sort"
	"sync"
	"github.com/c2fo/vfs/v6"
)
var mmu sync.RWMutex
var m map[string]vfs.FileSystem
func Register(name string, v vfs.FileSystem) {
	mmu.Lock()
	m[name] = v
	mmu.Unlock()
}
func Unregister(name string) {
	mmu.Lock()
	delete(m, name)
	mmu.Unlock()
}