Add assertions to verify that grants are unmapped
Fix grant unmapping in exec and exit, thus fixing some crashes without network cards

Keeping file handles (and pids, and scheme id, ...) as usize is a
footgun. Let's remove it.

Keeping pid (and file descriptor, and scheme id, ...) as usize is a
footgun. Let's remove it.

Keeping scheme ids (and pids, and file handles, ...) as usize is a
footgun. Let's remove it.