Ensure that relative operations handle broad API

[fsgeek]

Quite a few calls have an absolute variant and a relative variant. In the absolute variant, the path is passed in and interpreted from root to branch. In the relative variant, the path is passed in along with some sort of directory reference and the path is interpreted relative to that directory.

The API model around this has been evolving. Instead of having different APIs, I've chosen to keep the number of APIs smaller and instead pass in an (optional) reference to the parent.

This work item is to ensure:

• That any split APIs are merged together;
• That these APIs check for NULL as the passed-in parameter, in which case they must zero that in the communications message (when applicable). If the parent is passed in as a null/invalid parent reference, that should have the same effect.
• That the server side implementation is cognizant of that, obtains the correct inode number on the server side, and then passes that as the parent operation. There are other examples of this already (e.g., name map).