AbstractPollTask: log exceptional events

AbstractPollTask: log exceptional events

Previously, when AbstractPollTask receives a POLLERR or POLLNVAL event,
it would silently cancel the task. Now it will generate a message like
this:

   !!! SpawnProcess received strange poll event: 8

The message is displayed via a new _log_poll_exception method, which
is overridden by AbstractEbuildProcess to log the message via elog.

This might help diagnose reported cases of SIGTERM signals killing
ebuild processes for no apparent reason:

   http://code.google.com/p/chromium-os/issues/detail?id=12968