Support 'pinned' flag

[jimblandy]

Some applications need their counters enabled the entire period, or it's no use. These want to set the pinned bit in the perf_event_attr struct, which causes reads to return EOF if the counter wasn't scheduled.

[jimblandy]

It's also essential to be able to set this when creating groups. So just changing Builder isn't sufficient.

Also, this is not just about setting the bit in the perf_event_attr struct. The Group and Counter read methods also need to be prepared for zero-length reads, which is how the interface reports that it was unable to put the counter on the CPU.