Otherwise seeking into a raw dmg file will fail when hitting
parts of it that are not covered by a band. The expectation
in that case is to pad with zeroes, like we do for partial
bands.
Opening and closing files is bad for performance, so now that
we accurately track the situation of hitting too many open files
we can keep the band files open, even for the non-readbuf code
path.
This unifies the logic of opening (and caching file descriptors)
between the readbuf and non-readbuf code paths.
The previous logic was flawed anyways, as reading a range could
potentially span multiple bands, but we only checked if we were
just below the max limit.
