std.Io.Threaded: fix the cancellation race

Now, before a syscall is entered, beginSyscall is called, which may
return error.Canceled. After syscall returns, whether error or success,
endSyscall is called. If the syscall returns EINTR then checkCancel is
called.

`cancelRequested` is removed from the std.Io VTable for now, with plans
to replace it with a more powerful API that allows protection against
cancellation requests.

closes #25751

lib/std/Io.zig

@@ -620,11 +620,6 @@ pub const VTable = struct {
         result: []u8,
         result_alignment: std.mem.Alignment,
     ) void,
-    /// Returns whether the current thread of execution is known to have
-    /// been requested to cancel.
-    ///
-    /// Thread-safe.
-    cancelRequested: *const fn (?*anyopaque) bool,
 
     /// When this function returns, implementation guarantees that `start` has
     /// either already been called, or a unit of concurrency has been assigned