expand support for noreturn declarations

Although gcc specifies support for declaring a function as
noreturn after the function name and parameters, other compilers
do so via an attribute at the start of the declaration. Add some
macro support for the attribute-at-the-beginning method:
  o MS Visual Studio compiler
  o Upcoming C23 standard (untested at this point)

sys/vms/vmsmisc.c

@@ -9,14 +9,14 @@
 
 int debuggable = 0; /* 1 if we can debug or show a call trace */
 
-void vms_exit(int);
-void vms_abort(void);
+ATTRNORETURN void vms_exit(int);
+ATTRNORETURN void vms_abort(void);
 
 /* first arg should be unsigned long but <lib$routines.h> has unsigned int */
 extern void VDECL(lib$signal, (unsigned, ...));
 
 /* terminate, converting Unix-style exit code into VMS status code */
-void
+ATTRNORETURN void
 vms_exit(int status)
 {
     /* convert non-zero to failure, zero to success */
@@ -25,7 +25,7 @@ vms_exit(int status)
 }
 
 /* put the user into the debugger; used for abort() when in wizard mode */
-void
+ATTRNORETURN void
 vms_abort(void)
 {
     if (debuggable)

sys/windows/windmain.c

@@ -89,6 +89,8 @@ static struct stat hbuf;
 
 extern char orgdir[];
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 int
 get_known_folder_path(
     const KNOWNFOLDERID * folder_id,
@@ -128,6 +130,8 @@ create_directory(const char * path)
         error("Unable to create directory '%s'", path);
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 int
 build_known_folder_path(
     const KNOWNFOLDERID * folder_id,
@@ -1149,6 +1153,8 @@ eraseoldlocks(void)
     return (1);   /* success! */
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 int
 getlock(void)
 {
@@ -1288,6 +1294,8 @@ file_exists(const char* path)
     return TRUE;
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 /* 
   file_newer returns TRUE if the file at a_path is newer then the file
   at b_path.  If a_path does not exist, it returns FALSE.  If b_path