mount_alias() is broken

[mato]

mount_alias() in lib/fsu_mount.c has at least the following problems:

• it never registers al->fsa_path with rump_pub_etfs_register()
• it then passes al->fsa_path as the fsdevice into mount_struct(), it should be passing RUMPFSDEV
  found these while fixing the error reporting for PR fsu_ecp: Fix "Bad file descriptor" and file mode bits #13

It's not clear to me how the alias functionality is intended to work, is this a work in progress?

[anttikantee]

I can't quite remember, but IIRC mount_alias is supposed to somehow interact with FSU_* from the environment.

Can you provide test cases for triggering the problems?