reduce the scope of usage of off_t

Setting a _FILE_OFFSET_BITS define is not a guarantee off_t will be 64-bit large.

So I reduced the usage of off_t to the parts where it's really needed as such (lseek calls). In the other places the (internal) type we want is usually either an uint64_t (as in the access modules) or a size_t.

We no longer force the off_t size on mingw32. Keep its size as it is.

A new vlc_file_offset type is introduced. It could be used by the input access modules as well. Or we could just use uint64_t instead of a new type.