lib: Fix handling of poll

lib: Fix handling of poll

poll returns the number of revents that we need to handle
in the array.  revent is a bit field of events that need
to be handled.  thread.c was treating each sub item in the
bitfield as a separate item to handle.

As such the loop over the pollfds would quit early
sometimes.

Ticket: CM-10077
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>