wasi: introduce platform.Select and use it for poll_oneoff (#1346)

The PR introduces the `platform.Select()` API, wrapping `select(2)` on POSIX and emulated in some cases on Windows. RATIONALE.md contains a full explanation of the approach followed in `poll_oneoff` to handle Stdin and the other types of file descriptors, and the clock subscriptions.

It also introduces an abstraction (`StdioFilePoller`) to allow the simulation of different scenarios (waiting for input, input ready, timeout expired, etc.) when unit-testing interactive input.

This closes #1317.

Signed-off-by: Edoardo Vacchi <evacchi@users.noreply.github.com>

internal/platform/fdset_darwin.go

@@ -0,0 +1,8 @@
+package platform
+
+import "syscall"
+
+const nfdbits = 0x20
+
+// FdSet re-exports syscall.FdSet with utility methods.
+type FdSet syscall.FdSet