Add hooking functionality

This allows us to hook chosen functions and do interesting things there (in
particular: reentrancy checking).

test/unit/prof_tctx.c

@@ -41,6 +41,6 @@ TEST_END
 
 int
 main(void) {
-	return test(
+	return test_no_reentrancy(
 	    test_prof_realloc);
 }