sort of/kind of support PANICTRACE on VMS (trunk only)

I don't think this is useful enough to recommend ordinary users
enable it, but it's close enough to being useful that I don't want
to leave it to become subject to bit rot like umpteen other unfinished
patches.  Anyone running in wizard mode who has a panic already gets
pushed into the debugger on VMS, although it doesn't work for what might
be considered the most important configuration (a secure playground, as
opposed to the wide-open one I've always been content to leave mine at).

include/extern.h

@@ -2486,6 +2486,9 @@ E boolean NDECL(authorize_wizard_mode);
 
 E void NDECL(vms_abort);
 E void FDECL(vms_exit, (int));
+#ifdef PANICTRACE
+E void FDECL(vms_traceback, (int));
+#endif
 
 /* ### vmstty.c ### */
 

include/vmsconf.h

@@ -73,6 +73,20 @@
  */
 /* #define SECURE */
 
+/*
+ * If you use SECURE you'll need to link /noTraceback, in which case
+ * there's no point trying to get extra PANICTRACE info and this might
+ * as well be commented out.  When enabled, the sysconf file controls
+ * how to handle it (note that we're hijacking the Unix GDB setting):
+PANICTRACE_GDB=0  #behave as if PANICTRACE was disabled
+PANICTRACE_GDB=1  #at conclusion of panic, show a call traceback and exit
+PANICTRACE_GDB=2  #at conclusion of panic, show a call traceback and then
+ *                # remain in the debugger for more interactive debugging
+ *                # (not as useful as it might sound since we're normally
+ *                # linked /noDebug so there's no symbol table accessible)
+ */
+/* #define PANICTRACE */
+
 /*
  * Put the readonly data files into a single container rather than into
  * separate files in the playground directory.