Kill most warnings in lib/DtTerm.

Mostly missing headers, explicit parentheses and new prototypes.

Some Caveats:
* I haven't compile-tested the SVR4 getpty file, it might need another tweak
* There were operator precedence bugs in TermPrimCursor.c and TermPrimRender.c
  (^ vs. !=).  This might change behaviour, but at least I haven't experienced
  any crashes ...
* This adds a little more dependencies for include ordering, but unless we
  want to play the "headers that include headers that include headers..." game,
  this is unavoidable.

cde/lib/DtTerm/Term/TermFunctionKey.c

@@ -35,6 +35,7 @@ static char rcs_id[] = "$XConsortium: TermFunctionKey.c /main/1 1996/04/21 19:15
 
 #include "TermHeader.h"
 #include <Xm/Xm.h>
+#include "TermAction.h"
 #include "TermPrimI.h"
 #include "TermP.h"
 /* #include "TermFunctionKeyP.h" */
@@ -147,6 +148,7 @@ _DtTermFunctionKeyClear(
     }
 }
 
+void
 _DtTermFunctionKeyInit(
    Widget w
 )