[uaccess] Change userptr_t to be a pointer type

The original motivation for the userptr_t type was to be able to
support a pure 16-bit real-mode memory model in which a segment:offset
value could be encoded as an unsigned long, with corresponding
copy_from_user() and copy_to_user() functions used to perform
real-mode segmented memory accesses.

Since this memory model was first created almost twenty years ago, no
serious effort has been made to support a pure 16-bit mode of
operation for iPXE.  The constraints imposed by the memory model are
becoming increasingly cumbersome to work within: for example, the
parsing of devicetree structures is hugely simplified by being able to
use and return direct pointers to the names and property values.  The
devicetree code therefore relies upon virt_to_user(), which is
nominally illegal under the userptr_t memory model.

Drop support for the concept of a memory location that cannot be
reached through a straightforward pointer dereference, by redefining
userptr_t to be a simple pointer type.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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