[MAJOR] modularize the polling mechanisms

[MAJOR] modularize the polling mechanisms

select, poll and epoll now have their dedicated functions and have
been split into distinct files. Several FD manipulation primitives
have been provided with each poller.

The rest of the code needs to be cleaned to remove traces of
StaticReadEvent/StaticWriteEvent. A trick involving a macro has
temporarily been used right now. Some work needs to be done to
factorize tests and sets everywhere.