WasmFS: `newselect` system call

[tlively]

man page

[kripken]

I am not sure if we need this one with any urgency. It looks like musl never calls newselect (only the older version), and we also lack any testing of newselect. That is, the only possible use of this seems to be someone calling the syscall directly, and we've never considered that important enough to test.

[tlively]

Sounds good. I'll remove this from the WasmFS feature parity milestone since it's not a blocker.