MFC r305994 (by emaste@):

Catch up to sys/capability.h rename to sys/capsicum.h in r263232

MFC r297297:

When write(2) on eventfd object fails with the error EAGAIN do not return the number of bytes written.

MFC r297296:

Implement O_NONBLOCK flag via fcntl(F_SETFL) for eventfd object.

MFC r284626:

Add EPOLLRDHUP support.

MFC r283484:

Implement epoll_pwait() system call.

MFC r283470:

Add EPOLLERR flag handling to epoll.

MFC r283444:

Implement eventfd system call.

MFC r283441:

Implement epoll family system calls. This is a tiny wrapper
around kqueue() to implement epoll subset of functionality.
The kqueue user data are 32bit on i386 which is not enough for
epoll user data, so we keep user data in the proc emuldata.

Initial patch developed by rdivacky@ in 2007, then extended
by Yuri Victorovich @ r255672 and finished by me
in collaboration with mjg@ and jillies@.