Syscalls that wait on fds don't handle signal mask arguments

[stevenengler]

The syscalls that wait on fds often have versions that accept a signal mask. For example epoll_pwait, epoll_pwait2, pselect6, and ppoll. We support these syscalls in Shadow, but they don't handle the signal masks. For example epoll_pwait and epoll_pwait2 will return an error if the signal mask is non-null, while pselect6 and ppoll ignore the signal mask arguments completely.