Handle nested p* system calls if necessary

We are currently not able to handle - and won't be even after fix for #3336 (closed) - nested calls of ppoll, pselect6, epoll_pwait, e.g. if the app makes such a system call in the signal handler and the kernel supports it.

The future fix for #3336 (closed) will have to save the app's sigmask pre-syscall since we're not executing everything atomically. Nesting will not be supported.