feat(hypercall): introduce chdir and integrate it in UhyveFileMap

[n0toose]

Related to: hermit-os/kernel#1483
Hermit PR: hermit-os/kernel#1485

[n0toose]

This should allow the VM to change the guest "current working directory" stored in UhyveFileMap. This is an early concept, but this is what I'm going for:

1. During UhyveFileMap::new (with /root or UHYVE_MOUNTPATH or another interface to be decided) by default, file.txt (user input) gets added to the file map as /root/file.txt.
2. Hermit will be able to create /root/test_folder, chdir into it, then open a file called /root/test_folder/file.txt. In that case, OpenParams will contain file.txt, but we'll have stored /root/test_folder internally.
3. By opening file.txt, a mapping with the guest path /root/test_folder/file.txt should be added.

The current design assumes that open in Hermit will continue to work as-is - by not providing Uhyve with an absolute path -, so Uhyve tries to prepend the chdir with every given opportunity itself. This has not been tested extensively yet.

Partially fixes #815.

[jounathaen]

Suggested change

	/// Parameters for a [`FileUnlink`](crate::Hypercall::Chdir) hypercall.
	/// Parameters for a [`ChangeDir`](crate::Hypercall::ChangeDir) hypercall.