Kernel: Pass all user pointers as Userspace<T*> in syscalls

[Hendiadyoin1]

As the commit title suggests the main change is always passing the pointer arguments as Userspace<T*>.
This commit also makes creations of Userspace pointers more explicit on the kernel side and handles all the fallout from those changes.

These changes include:

• Adding dereferenceability on the Userland side of Userspace<T*>
• Adding const up conversion to Userspace<T*>
• changing kernel/user copies to use the correct overloads
• Adding/removing casts as necessary
• Changing signatures as necessary

And a bit more controversial:

• Changing futex params to contain volatile pointers
  This was the easiest way of getting the correct types for the atomic operations in sys$futex. The Kernel APIs should likely change signature instead, as volatility and atomicity are two very different concepts in modern c++.

[spholz]

Can you please split this up into smaller atomic commits? (Maybe not one per syscall though.) There also seem to be some smaller drive-by changes between purely mechanical changes. It would be nice if those could be separate commits as well.

(I kind of liked that we didn't use too many fancy C++ features over the syscall border. But I guess it's worth the extra safety here.)

[spholz]

The Kernel APIs should likely change signature instead, as volatility and atomicity are two very different concepts in modern c++.

Why? Have a look at all the atomic* functions and builtins. They also use volatile parameters. "atomics" don't exist as a type at the language level.