Various fixes for unseekable, unmmapable, and non-normal files, so that fallback to read() rather than mmap() works.

Original commit message from CVS:
Various fixes for unseekable, unmmapable, and non-normal files, so that
fallback to read() rather than mmap() works.

Allow newsegment events with start == end, so that cases where that's
correct work (e.g. filesrc on a zero-size file).