Files should explicitly include the definitions they directly depend on (#355)

... start by fixing the include files and the fallout in C source from
removing includes not directly needed by headers that previously included
them.

inc/osmsgdefs.h

@@ -1,5 +1,6 @@
 #ifndef OSMSGDEFS_H
 #define OSMSGDEFS_H 1
+#include "lispemul.h" /* for LispPTR */
 void mess_init(void);
 void mess_reset(void);
 LispPTR mess_read(LispPTR *args);