uint32: more longs converted to proper types

I also moved the MAC struct over to the mac.h header file and made sure
that the users of that struct include that file.

src/session.c

@@ -55,6 +55,7 @@
 #include "transport.h"
 #include "session.h"
 #include "channel.h"
+#include "mac.h"
 
 /* libssh2_default_alloc
  */