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/TermPrim/TermPrimRenderMb.c

@@ -48,8 +48,10 @@ static char rcs_id[] = "$XConsortium: TermPrimRenderMb.c /main/1 1996/04/21 19:1
 #include "TermPrimOSDepI.h"
 #include "TermPrimBufferP.h"
 #include "TermPrimRenderP.h"
+#include "TermPrimSelect.h"
 #include "TermPrimSelectP.h"
 #include <limits.h>
+#include <wchar.h>
 
 void
 _DtTermPrimRefreshTextWc(Widget w, short startColumn, short startRow,