Add more epoll fd flag coverage

[metan-ucw]

As it is now we only test the very basic flags for epoll events, so far there seems to be only tests that do work with EPOLLIN and EPOLLOUT, but there is much more for instance EPOLLRDHUP, EPOLLHUP, EPOLLERR, EPOLLET, EPOLLONESHOT, etc.

Recently there was a regression with EPOLLET and pipes:

https://lwn.net/Articles/864947/ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3a34b13a88ca

[acerv]

I will take care of this ticket

[coolgw]

@acerv what's current plan? :)

[acerv]

@coolgw started, patch was reviewed, but it needs to be adapted. This is a low priority task, since epoll is an already well tested feature in the kernel and it's really hard it breaks