MFC r283492:

Implement Linux specific syncfs() system call.

MFC r283488:

Implement recvmmsg() and sendmmsg() system calls.

MFC r283484:

Implement epoll_pwait() system call.

MFC r283480:

Add utimensat() system call.

MFC r283465:

Add preliminary fallocate system call implementation
to emulate posix_fallocate() function.

MFC r283451:

Implement ppoll() system call.

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@.