epoll: make epoll-poll event conversion more explicit

epoll and poll() use different event names (e.g., EPOLLIN vs. POLLIN),
but in practice the bits are identical. Make this conversion more
explicit, and add static_assert()s that the bits are indeed identical.

We still have dynamic assert() that the bits we don't support -
EPOLLET (for edge triggered behavior) and EPOLLONESHOT - aren't
used.

This patch only makes the code cleaner, but doesn't change anything
in its behavior.`