dtfile: Add missing prototypes

Add missing prototypes and header files to the dtfile
code in order to move closer towards 64-bit compatibility.

Extract the following functions from Motif internal headers:

_XmGetWidgetExtData
_XmRecordEvent
_XmStringUngenerate
_XmTextFieldSetDestination
_XmGetActiveTopLevelMenu

Extract manually prototypes of the obsolete Motif interface:
_XmHighlightBorder
_XmUnhighlightBorder

Remove XmPrivate.h if extractprototype.awk fails

Make the following header files available via -I:
	codelibs/boolean.h
	codelibs/pathutils.h
	codelibs/shellutils.h
and remove shellutils.h from dtwm directory.

cde/programs/dtfile/dtcopy/sharedFuncs.c

@@ -96,6 +96,7 @@
 
 #include <Dt/Connect.h>
 #include <Dt/DtNlUtils.h>
+#include <Dt/SharedProcs.h>
 
 #include "sharedFuncs.h"
 #include "dtcopy.h"
@@ -645,7 +646,7 @@ _DtCopyPathFromInput(input_string, current_dir)
     else if (!path)
     {
         XtFree(tmp_path);
-        return;
+        return NULL;
     }
 
     /* Resolve '.' or '..' -- input is overwritten, output may be NULL! */