volatile-mode: use the usual string table macros for implementing volatile_mode_from_string()

Let's shorten our code a bit, and also add the matching _to_string()
call.

src/shared/volatile-util.h

@@ -16,5 +16,6 @@ typedef enum VolatileMode {
 } VolatileMode;
 
 VolatileMode volatile_mode_from_string(const char *s);
+const char* volatile_mode_to_string(VolatileMode m);
 
 int query_volatile_mode(VolatileMode *ret);