fix calloc() calls

* size_t nmemb (number of elements) first, then size_t size
* do not assume char is size 1 but use sizeof()
* allocate for char, not pointer to char (found by Michael McConville,
  fixes #261)

darwin/DarwinProcess.c

@@ -39,7 +39,7 @@ ProcessClass DarwinProcess_class = {
 };
 
 DarwinProcess* DarwinProcess_new(Settings* settings) {
-   DarwinProcess* this = calloc(sizeof(DarwinProcess), 1);
+   DarwinProcess* this = calloc(1, sizeof(DarwinProcess));
    Object_setClass(this, Class(DarwinProcess));
    Process_init(&this->super, settings);