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/dthello/Imakefile

@@ -17,9 +17,10 @@ SYS_LIBRARIES = -lm -ldl -lgen
    SRCS1 = dthello.c
    SRCS = $(SRCS1)
    OBJS = dthello.o \
-          $(CDELIBSRC)/DtSvc/DtUtil2/EnvControl.o \
           $(CDELIBSRC)/DtSvc/DtUtil2/DtNlUtils.o \
-          $(CDELIBSRC)/DtSvc/DtUtil2/GetDispRes.o
+          $(CDELIBSRC)/DtSvc/DtUtil2/EnvControl.o \
+          $(CDELIBSRC)/DtSvc/DtUtil2/GetDispRes.o \
+          $(CDELIBSRC)/DtSvc/DtUtil2/MsgCat.o
 
 ComplexProgramTarget(dthello)