Add %option nounput, since we no longer use unput.

Add %option nounput, since we no longer use unput.
(unexpected_eof): Renamed from unexpected_end_of_file, for brevity.
Do not insert the expected token via unput, as this runs afoul
of a POSIX-compatibility bug in flex 2.5.31.
All uses changed to BEGIN the parent state,
since we no longer insert the expected token via unput.