[umalloc] Remove userptr_t from user memory allocations

Use standard void pointers for umalloc(), urealloc(), and ufree(),
with the "u" prefix retained to indicate that these allocations are
made from external ("user") memory rather than from the internal heap.

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

src/tests/umalloc_test.c

@@ -1,12 +1,11 @@
 #include <stdio.h>
-#include <ipxe/uaccess.h>
 #include <ipxe/umalloc.h>
 #include <ipxe/io.h>
 
 void umalloc_test ( void ) {
 	struct memory_map memmap;
-	userptr_t bob;
-	userptr_t fred;
+	void *bob;
+	void *fred;
 
 	printf ( "Before allocation:\n" );
 	get_memmap ( &memmap );