Fix a few more harmless compiler warnings:

- myfree in pager.c was called with an argument, so add one to the
dummy definition
- pgoff is offset_t (unsigned) and doesn't need to be compared to 0
- clang says '*(int *)0 = 0' will be optimized away instead of keeping
the segfault without a volatile hint (?! that is wrong!), but it causes
no harm to add anyway.

kernel/pager.c

@@ -161,7 +161,7 @@ myalloc(struct swapinfo *si, size_t sz)
 }
 
 void
-myfree()
+myfree(void *p)
 {
 	/* nothing so far */
 }