unix: add flag for blocking SIGPROF during poll

Add a per-event loop flag for blocking SIGPROF signals when polling for
events.

The motivation for this addition is to reduce the number of wakeups and
subsequent clock_gettime() system calls when using a sampling profiler.

On Linux, this switches from epoll_wait() to epoll_pwait() when enabled.
Other platforms bracket the poll syscall with pthread_sigmask() calls.

R=@saghul?

No tests yet.  It's kind of hard to write one that is not timing sensitive.
UV__FOO since its internal perhaps?