inserting: treat also the final \r of a Mac file as a newline

Until now (when not leaving files unconverted), nano would fumble and
drop the final carriage return of a Mac file, and would thus treat the
last line of such a file as an unterminated line and prepend it to the
current line of the buffer.  Correct that, and delete the dead piece
of code that was meant to do this.

This fixes https://savannah.gnu.org/bugs/?47716.
1 file changed