lxc-copy: cleanup

lxc-copy: cleanup

- make free_mnts() work directly on the globals mnt_table and mnt_table_size
- have free_mnts() set mnt_table = NULL and mnt_table_size = 0 when its done to
  avoid double frees
- simplify error-handling in do_clone_ephemeral()
- do_clone_ephemeral(): when chmod() falls to set permissions on the temporary
  folder we created for mkdtemp() remove the folder
- simplify error handling in main()

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>