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/serialVFD.c

@@ -505,7 +505,7 @@ serialVFD_num( Driver * drvthis, int x, int num )
 		p->ccmode = CCMODE_BIGNUM; // Switch customcharactermode to bignum.
 	}
 	// Lib_adv_bignum does everything needed to show the bignumbers.
-	lib_adv_bignum(drvthis, x, num, do_init, p->customchars);
+	lib_adv_bignum(drvthis, x, num, 0, do_init);
 }