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/TermBuffer.c

@@ -39,6 +39,7 @@ static char rcs_id[] = "$TOG: TermBuffer.c /main/2 1997/04/17 18:04:41 samborn $
 #define	USE_MEMCPY	/* use memcpy for line movement... */
 
 #include <Xm/Xm.h>
+#include "TermPrim.h"
 #include "TermHeader.h"       /* for MIN/MAX */
 #include "TermBufferP.h"
 #include "TermEnhance.h"
@@ -46,7 +47,7 @@ static char rcs_id[] = "$TOG: TermBuffer.c /main/2 1997/04/17 18:04:41 samborn $
 /*
 ** clear "count" enhancements starting at startCol
 */
-static
+static void
 clearEnhancements
 (
     TermBuffer tb,
@@ -451,7 +452,7 @@ _DtTermBufferFreeBuffer
 /*
 ** clear all the enhancements from startCol through stopCol
 */
-static
+static void
 clearEnhancements
 (
     TermBuffer tb,