Centralize catgets() calls through MsgCat

CDE has relied upon catgets() implementations following a relaxed
interpretation of the XPG internationalization standard that ignored
-1, the standard error value returned by catopen, as the catalog
argument. However, this same behavior causes segmentation faults with
the musl C library.

This patch:

- Centralizes (with the exception of ToolTalk) all calls to catopen(),
  catgets(), and catclose() through MsgCat within the DtSvc library.
- Prevents calls to catgets() and catclose() that rely upon
  undefined behavior.
- Eliminates a number of bespoke catgets() wrappers, including multiple
  redundant caching implementations designed to work around a design
  peculiarity in HP/UX.
- Eases building CDE without XPG internationalization support by providing
  the appropriate macros.

cde/programs/dtappbuilder/src/ab/dnd_ed_stubs.c

@@ -656,7 +656,7 @@ do_auto_apply(
     {   
     	if (changing_objects) 
     	{
-    	    sprintf(buffer, catgets(Dtb_project_catd, 100, 31,
+	    sprintf(buffer, CATGETS(Dtb_project_catd, 100, 31,
 		"Drag and drop properties for \"%s\"\n\
 		have been modified but not Applied.\n\n\
 		You can Apply the Changes or Cancel the\n\
@@ -666,7 +666,7 @@ do_auto_apply(
     	{
 	    if(new_obj != (ABObj) NULL) 
 	    {
-    	    	sprintf(buffer, catgets(Dtb_project_catd, 100, 32,
+		sprintf(buffer, CATGETS(Dtb_project_catd, 100, 32,
 		"Drag and drop properties for \"%s\"\n\
 		have been modified but not Applied.\n\n\
 		You can Apply the Changes or Cancel the\n\
@@ -674,7 +674,7 @@ do_auto_apply(
 	    }
 	    else 
 	    {
-    	    	sprintf(buffer,catgets(Dtb_project_catd, 100, 33,
+		sprintf(buffer,CATGETS(Dtb_project_catd, 100, 33,
 		"Drag and drop properties for \"%s\"\n\
 		have been modified but not Applied.\n\n\
 		You can Apply the Changes or Cancel the\n\