HandleTable::add_handle should error if the table is full.

[todo]

HandleTable::add_handle currently assumes that insertion will always succeed. It does not implement any handle count limitations present in Horizon/NX. Furthermore, if the handle table is completely filled (e.g. there are 2^32 handles), the function will infinite loop. And as if that wasn't enough: it doesn't technically guarantee a handle will not get reused.

---

https://github.com/roblabla42/KFS/blob/adee47274ba5c0dc0ef112ce8933becd63d3da77/kernel/src/process.rs#L259-L269

---

This issue was generated by todo based on a TODO comment in adee472 when #90 was merged. cc @roblabla.