sd-bus: add define for the maximum signature length

Less magic numbers in the code…

src/libsystemd/sd-bus/bus-message.c

@@ -284,7 +284,7 @@ static int message_append_field_signature(
         /* dbus1 doesn't allow signatures over 8bit, let's enforce
          * this globally, to not risk convertability */
         l = strlen(s);
-        if (l > 255)
+        if (l > SD_BUS_MAXIMUM_SIGNATURE_LENGTH)
                 return -EINVAL;
 
         /* Signature "(yv)" where the variant contains "g" */

src/libsystemd/sd-bus/bus-signature.c

@@ -144,5 +144,5 @@ bool signature_is_valid(const char *s, bool allow_dict_entry) {
                 p += t;
         }
 
-        return p - s <= 255;
+        return p - s <= SD_BUS_MAXIMUM_SIGNATURE_LENGTH;
 }