refactor adv_bignum:

* support display heights > 4 (treated like height = 4);
* determine number of user-defineable chars using get_free_chars();
* allow custom chars starting at a character position (offset) != 0
* update and extend documentation

server/drivers/hd44780.c

@@ -712,7 +712,7 @@ HD44780_num (Driver *drvthis, int x, int num)
 	}
 
 	// Lib_adv_bignum does everything needed to show the bignumbers.
-	lib_adv_bignum(drvthis, x, num, do_init, NUM_CCs);
+	lib_adv_bignum(drvthis, x, num, 0, do_init);
 }