sysctl-util: introduce sysctl_write_verify()

This is useful when writing coredump pattern. But the logic itself is
quite generic. Let's add the helper in our basic library.
Preparation for later change.

src/basic/sysctl-util.h

@@ -12,6 +12,7 @@ int sysctl_writef(const char *property, const char *format, ...) _printf_(2, 3);
 static inline int sysctl_write(const char *property, const char *value) {
         return sysctl_write_full(property, value, NULL);
 }
+int sysctl_write_verify(const char *property, const char *value);
 
 int sysctl_read_ip_property(int af, const char *ifname, const char *property, char **ret);
 int sysctl_read_ip_property_int(int af, const char *ifname, const char *property, int *ret);