vm: replace phys avl by array

. make vm be able to use malloc() by overriding brk()
   and minix_mmap() functions
 . phys regions can then be malloc()ed and free()d instead
   of being in an avl tree, which is slightly faster
 . 'offset' field in phys_region can go too (offset is implied
   by position in array) but leads to bigger code changes

lib/libc/stdlib/malloc.c

@@ -95,6 +95,13 @@ void utrace(struct ut *, int);
 #   define _MALLOC_UNLOCK()		if (__isthreaded) _SPINUNLOCK(&thread_lock);
 #endif /* __FreeBSD__ */
 
+/* #undef these things so that malloc uses the non-internal symbols.
+ * This is necessary for VM to be able to define its own versions, and
+ * use this malloc.
+ */
+#undef minix_mmap
+#undef minix_munmap
+
 #include <assert.h>
 
 #include <sys/types.h>