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/dtexec/Imakefile

@@ -6,11 +6,12 @@ DEPEND_DEFINES = $(DEPENDDEFINES)
 DEFINES = -DMSGLOG_CLIENT_ONLY
 INCLUDES = -I. $(TIRPCINC)
 
-SRCS = DtGetMessage.c Main.c MsgLog.c
-OBJS = DtGetMessage.o Main.o MsgLog.o
+SRCS = DtGetMessage.c Main.c MsgCat.c MsgLog.c
+OBJS = DtGetMessage.o Main.o MsgCat.o MsgLog.o
 
 ComplexProgramTarget(dtexec)
 
 LinkSourceFile(DtSvcLock.h,$(DTSVCSRC)/include)
+LinkSourceFile(MsgCat.c,$(DTSVCSRC)/DtUtil2)
 LinkSourceFile(MsgLog.c,$(DTSVCSRC)/DtUtil2)
 LinkSourceFile(DtGetMessage.c,$(DTSVCSRC)/DtUtil2)