Add methods for linking a keyring to another keyring

[afontenot]

This is an important keyctl operation that can have implications on whether a key is readable and shows up in Search operations.

It may not be immediately be apparent that this is a supported operation when calling KEYCTL_LINK, you have to dig through the docs to find this.

man 7 keyrings:

As previously mentioned, keyrings are a special type of key that
contains links to other keys (which may include other keyrings). Keys may be linked to by multiple keyrings.

[afontenot]

This one is a little thrown together, I'm willing to rework it and do it differently (e.g. we could make it possible to convert a KeyRingIdentifier to a KeySerialId more explicitly than KeySerialId::new(KeyRingIdentifier as i32) and that might be sufficient for these use cases with a little adjustment to the docs to make clear how to do this.

I do think there does need to be a supported way to link keyrings.

Also willing to add tests and improve the comments if you can do a cursory review and say what's needed.

[landhb]

@afontenot Thanks for the PR!

I think your explicit methods are better than having users convert to a raw ID and using existing Key methods to workaround it.

It looks clean, some tests would be great though.

I'll try to get to these PRs this weekend.

[afontenot]

@landhb tests have been added.

Implementing set_perms on KeyRing in the future would be useful, it would also allow testing some of the claims made about e.g. write / linking access (throughout the KeyRing implementation, not specific to this commit).