[bdf] Clean up the atom property parsing.

* src/bdflib.c (bdf_is_atom_): Refactor code with fewer checks.
(bdf_list_join_): Return NULL.
(bdf_add_comment_): Use const argument.
(bdf_get_property): Ditto, ditto, make the function static.
* src/bdf.h (bdf_get_property): Remove prototype.
2 files changed