Change type of uspace pointers in kernel from pointer type to numeric type
From kernel's perspective, userspace addresses are not valid pointers,
and can only be used in calls to copy_to/from_uspace().
Therefore, we change the type of those arguments and variables to
uspace_addr_t which is an alias for sysarg_t.
This allows the compiler to catch accidental direct accesses to
userspace addresses.
Additionally, to avoid losing the type information in code,
a macro uspace_ptr(type) is used that translates to uspace_addr_t.
I makes no functional difference, but allows keeping the type information
in code in case we implement some sort of static checking for it in the future.
