Move CRLF input processing into ttywrite

This also allows us to remove the crlf field from the Key struct, since
the only difference it made was converting "\r" to "\r\n" (which is now
done automatically in ttywrite).  In addition, MODE_CRLF is no longer
referenced from x.c.

Signed-off-by: Devin J. Pohly <[email protected]>