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.h

@@ -70,7 +70,7 @@ _DtTermFunctionKeyClear(
 );
 
 
-extern _DtTermFunctionKeyInit(
+extern void _DtTermFunctionKeyInit(
    Widget w
 );