util-lib: move shall_restore_state() to shared/reboot-util

It's just a small function, but it is higher-level functionality.
I don't see a good place for it, reboot-util.[ch] seems least bad

src/basic/proc-cmdline.h

@@ -27,8 +27,6 @@ int proc_cmdline_get_key_many_internal(ProcCmdlineFlags flags, ...);
 char *proc_cmdline_key_startswith(const char *s, const char *prefix);
 bool proc_cmdline_key_streq(const char *x, const char *y);
 
-int shall_restore_state(void);
-
 /* A little helper call, to be used in proc_cmdline_parse_t callbacks */
 static inline bool proc_cmdline_value_missing(const char *key, const char *value) {
         if (!value) {