proc-cmdline: add runlevel to target mapping

src/basic/proc-cmdline.h

@@ -26,3 +26,4 @@ int parse_proc_cmdline(int (*parse_word)(const char *key, const char *value));
 int get_proc_cmdline_key(const char *parameter, char **value);
 
 int shall_restore_state(void);
+const char* runlevel_to_target(const char *rl);