Add initial version of renumber backend

Renumber is a way of renumbering container locks after the number
of locks available has changed.

For now, renumber only works with containers.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/lock/shm_lock_manager_linux.go

@@ -71,6 +71,11 @@ func (m *SHMLockManager) RetrieveLock(id uint32) (Locker, error) {
 	return lock, nil
 }
 
+// FreeAllLocks frees all locks in the manager
+func (m *SHMLockManager) FreeAllLocks() error {
+	return m.locks.DeallocateAllSemaphores()
+}
+
 // SHMLock is an individual shared memory lock.
 type SHMLock struct {
 	lockID  uint32