When too many chars are read by ReadChars() and we trim the limits to
get it right on the next pass, don't forget the TCL_UTF_MAX padding
demanded by Tcl_ExternalToUtf().  (Thanks for finding that, aku!)
Fix the factorPtr management.  It was just totaly wrong. The factor should
be a ratio of the record of bytes read to the record of chars read.
With those fixes, new test io-12.6 covers the "too many chars" code.
merge fork