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/dtdspmsg/Imakefile

@@ -1,5 +1,6 @@
 XCOMM $XConsortium: Imakefile /main/8 1996/10/09 14:04:16 drk $
 
+DEFINES = -DNO_XLIB
 SYS_LIBRARIES = -lm $(DYNLIBSYSLIB) $(REGEXSYSLIB)
 
 #ifdef HPArchitecture
@@ -9,6 +10,7 @@ EXTRA_DEFINES = +e
 PROGRAMS=dtdspmsg
 
 SRCS =  dtdspmsg.c
-OBJS =  dtdspmsg.o
+OBJS =  dtdspmsg.o MsgCat.o
 
 ComplexProgramTarget($(PROGRAMS))
+LinkSourceFile(MsgCat.c,$(DTSVCSRC)/DtUtil2)