#33 backport unput_yytext() to bespin

[Jiri Baum]

On newer versions of flex, unput_yytext() is not
allowed where we're using it. To fix this, we need to
replace every ocurrence of unput_yytext() with
yyless(0) in the iec.flex file.

This has already been done in the development version
(over a year ago) but needs to be back-ported to bespin.