[mounts] Allow only one-to-one uid/gid mappings for mounts

[andrei-toterman]

Describe the bug When specifying uid/gid mappings for a mount, you can use the same uid/gid in more than one mapping. This creates confusion as it is not clear which map should be used.

To Reproduce How, and what happened?

1. multipass mount dir instance:dir -u 1000:2000 -u 1000:3000 -u 3000:2000
2. Now it's not clear if a host file owned by 1000 should be owned 2000 or 3000 in the VM or if a remote file owned by 2000 should be owned by 1000 or 3000 on the host.

Expected behavior Throw an error when an attempted mount does not have a one-to-one correspondence between its mapped uids/gids.