MINOR: activity: rename confusing poll_* fields in the output

MINOR: activity: rename confusing poll_* fields in the output

We have poll_drop, poll_dead and poll_skip which are confusingly named
like their poll_io and poll_exp counterparts except that they are not
per poll() call but per-fd. This patch renames them to poll_drop_fd(),
poll_dead_fd() and poll_skip_fd() for this reason.