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.

Refs strongloop/strong-agent#3 and strongloop-internal/scrum-cs#37.
unix, windows: add uv_loop_configure() function

The only supported option right now is UV_LOOP_BLOCK_SIGNAL, which only
supports the SIGPROF signal and only on UNIX platforms.  So yes, it is
kind of limited right now.  But everything has to start somewhere.

Refs strongloop/strong-agent#3 and strongloop-internal/scrum-cs#37.