vfs: File.open() needs to do a seek(0) on the cached FileReader.

vfs: File.open() needs to do a seek(0) on the cached FileReader.

Otherwise if you open a file, read through it, and close it, then do it
again, you'll get zero bytes the second time.

To make this efficient, change seek() to not discard its _chunkiter every
single time; instead, keep the _chunkiter around until trying to read() from
a location that *isn't* the current offset.  Now seeking around in the file
is cheap.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>