Unix signals: fix polling for blocked signals via signalfd, fix handling of ignored signals

[francescolavra]

This change set fixes two issues in the Unix signal implementation:

• If a poll request is issued on a signalfd file descriptor to poll for the delivery of a blocked signal (i.e. a signal that is in the process signal mask), the poll syscall does not return when the signal is raised
• Ignored signals are not detectable via sigwaitinfo(), sigtimedwait() or sigwait(), nor via signalfd