Expose full epoll_ctl functionality.

Now the user has access to all struct elements in EpollEvent when
using epoll_ctl. This is in particular very intersting when the
user wants to access the ptr field.

The user can use GCHandle.ToIntPtr to give the struct a GCHandle
and then handle the pointer later on when using epoll_wait.
Merge pull request #1014 from txdv/master

Expose full epoll_ctl functionality.