Existing patches to make bigfile fail softly.

This is the existing C code patches for local modifications to the UCSC and CPAN bigfile libraries, to make them not fail so horribly when a track is unavailable. They are already live, but are just hanging around in my home directory. After talking with Anne, we decided that this is the best place for them. It's ok for them just to go onto master as they're only needed at compile-time.