Create/open file on WASI targets should have POLL_FD_READWRITE rights

[achan1989]

Fixes #14324

No testing performed. Original issue says "you can see this plainly in the source" -- agreed.

[squeek502]

It might be nice to include comments with quotes from the documentation.

[achan1989]

It might be nice to include comments with quotes from the documentation.

Rather than annotate each of the call sites, I could instead add the documentation once to the rights_t struct in std/os/wasi.zig ? If so, I'll do the rest of the fields too.

[squeek502]

The only intention I had with the inline comments is to note that POLL_FD_READWRITE is contingent on the FD_WRITE/FD_READ rights, i.e. it only grants an extra read-related privilege when FD_READ is also set, and it only grants an extra write-related privilege when FD_WRITE is set. Just seems like the sort of info that would be useful to have when looking at this bit of code in the future.

Not super important, though, and doc comments in rights_t might work just as well.

[andrewrk]

Thanks.