std.Io.Threaded: move `max_iovecs_len` to `Io`

[andrewrk]

Extracted from #25592.

Rather than having this be an inscrutible internal detail, it may be beneficial to expose this value fon inspection by making it public and available on std.Io interface:

const max_iovecs_len = 8;

Furthermore, it may be beneficial to make it one of the configurable std.Options.