This extracts a utility `syscallfs.ReaderAtOffset()` to allow WASI and
gojs to re-use the same logic to implement `syscall.Pread`.
What's different than before is that if WASI passes multiple iovecs an
emulated `ReaderAt` will seek to the read position on each call to
`Read` vs once per loop. This was a design decision to keep the call
sites compatible between files that implement ReaderAt and those that
emulate them with Seeker (e.g. avoid the need for a read-scoped closer/
defer function). The main use case for emulation is `embed.file`, whose
seek function is cheap, so there's little performance impact to this.
Signed-off-by: Adrian Cole <adrian@tetrate.io>
713e187796