Splitting keyfile handling in two "reader.c" and "writer.c" files
is not helpful. What is most interesting, is to see how property XYZ
is serialized to keyfile, and to verify that the parser does the
inverse. For that, it's easier if both the write_xzy() and parse_xyz()
function are beside each other, and not split accross files.

The more important reason is, that both reader and writer have their
separate handler arrays, for special handling of certain properties:
@key_parsers and @key_writers. These two should not be separate but will
be merged. Since they reference static functions, these functions must
all be in the same source file (unless, we put them into headers, which
would be unnecessary complex).

No code was changed, only moved.