ds: get rid of redundant and unnecessary POLL* constants

ds: get rid of redundant and unnecessary POLL* constants

EPOLL* constants already match their POLL* counterparts and
there's no way Linux can ever diverge or change the values
of those constants.  So we'll favor the EPOLL* ones since we
use EPOLLEXCLUSIVE, already.

For weird stuff like kqueue, we'd need to keep maintaining
the mapping, anyways.