feature: expand support for `linkat(2)`

[cyphar]

At the moment, we only support linking between two paths within the same root, with the paths being resolved during the pathrs_inroot_hardlink operation. This is much easier to implement than alternatives but there are a few usecases that are not handled by this:

• feature: support hardlinking fds into Roots #355
• feature: support a custom dirfd when hardlinking files #356