Split variable declaration out of debug_decl into debug_decl_vars()

so we can use it in main() when we know sudo_debug_enter() cannot
succeed.

src/sudo.c

@@ -152,7 +152,7 @@ main(int argc, char *argv[], char *envp[])
     struct sudo_settings *settings;
     struct plugin_container *plugin, *next;
     sigset_t mask;
-    debug_decl(main, SUDO_DEBUG_MAIN)
+    debug_decl_vars(main, SUDO_DEBUG_MAIN)
 
     /* Make sure fds 0-2 are open and do OS-specific initialization. */
     fix_fds();