convert.c: restructure the attribute checking part.

This separates the checkattr() call and interpretation of the
returned value specific to the 'crlf' attribute into separate
routines, so that we can run a single call to checkattr() to
check for more than one attributes, and then interprete what
the returned settings mean separately.

Signed-off-by: Junio C Hamano <junkio@cox.net>