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/dtlogin/Imakefile

@@ -166,7 +166,8 @@ INCLUDES = -I$(XPROJECTROOT)/include/freetype2
                   dpylist.o error.o file.o mitauth.o protodpy.o policy.o \
                   reset.o resource.o server.o session.o socket.o util.o \
                   verify.o sysauth.o fontpath.o $(DESOBJS) qualify.o choose.o \
-		  netaddr.o xdmcp.o $(PAM_OBJS) $(SOLARIS_OBJS)
+		  netaddr.o xdmcp.o $(PAM_OBJS) $(SOLARIS_OBJS) \
+		  $(CDELIBSRC)/DtSvc/DtUtil2/MsgCat.o
 
           PROG2 = dtgreet
 #ifdef AlphaArchitecture