Fixed failures in PAM module

Now the user is persented with help when they try to access a keyring
that isn't theirs or try to use fscrypt without a user keyring linked
into the session keyring.

Chaning the --user flag to (optionally) check for a proper keyring setup
allows us to fail early in cases where we need a working keyring.

Now instead of spawning a seperate thread we alternate between changing
the euid and ruid to both find the keyring and link it to the process
keyring. Note that we also ensure that the user keyring is linked into
the root keyring whenever possible.

This was creating an issue becasuse fully dropping privileges required
spawning a goroutine and using rutime.DropOSThread().

Now the offending panic will just be logged and the module will fail.
This is important as to not crash the login process.

Changes to the keyrings interface, corresponding UI changes, and misc changes

Use `/dev/disk/by-uuid` to get UUID links to other filesystems