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