spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

copyright: chase my name and email change

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37945

Expose eventfd in the native API/ABI using a new __specialfd syscall

eventfd is a Linux system call that produces special file descriptors
for event notification. When porting Linux software, it is currently
usually emulated by epoll-shim on top of kqueues. Unfortunately, kqueues
are not passable between processes. And, as noted by the author of
epoll-shim, even if they were, the library state would also have to be
passed somehow. This came up when debugging strange HW video decode
failures in Firefox. A native implementation would avoid these problems
and help with porting Linux software.

Since we now already have an eventfd implementation in the kernel (for
the Linuxulator), it's pretty easy to expose it natively, which is what
this patch does.

Submitted by: greg@unrelenting.technology
Reviewed by: markj (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26668