Add all file operation event notifications required by VFSForGit

[chrisd8088]

Specifically, we likely need to add event notifications for the FileModifed and HardLinkCreated operations; see the use of these in, e.g., the macOS and generic handlers registered by the GVFS provider.

We should likely map these to Linux equivalents as PROJFS_MODIFY and PROJFS_CREATE_SELF | PROJFS_ONLINK, where the ONLINK flag is used like the ONDIR one:

#define PROJFS_MODIFY 0x00000002

#define PROJFS_ONLINK himask(0x1000)

We're trying to align with fanotify.h in all this, but it's not perfect (yet!), and we're also tracking Amir Goldstein's superblock root watch work.

[chrisd8088]

Closed by #69.