cap-list: add capability_set_{from_string,to_string_alloc}()

src/basic/cap-list.h

@@ -22,3 +22,6 @@
 const char *capability_to_name(int id);
 int capability_from_name(const char *name);
 int capability_list_length(void);
+
+int capability_set_to_string_alloc(uint64_t set, char **s);
+int capability_set_from_string(const char *s, uint64_t *set);