Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

kqueue: Add EV_KEEPUDATA flag

When this flag is set, operations that update an existing kevent will
not change the udata field. This can be used to NOTE_TRIGGER or
EV_{EN,DIS}ABLE events without overwriting the stashed pointer.

Reviewed by: Domagoj Stolfa <domagoj.stolfa@gmail.com>
Obtained from: CheriBSD
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D30286

Fix various warnings in tests/sys/kqueue and bump WARNS

Reviewed By: kevans
Differential Revision: https://reviews.freebsd.org/D24296

kqueue tests: fix -fno-common build

vnode_fd and kqfd are both shared among multiple CU; define them exactly
once.

In the case of vnode_fd, it was simply the declaration that needed
correction.

-fno-common will become the default in GCC10/LLVM11.

MFC after: 3 days

Make whitespace more consistent in libkqueue tests.

After r337820, which "corrected" some spaces-instead-of-tab whitespace
issues in the libkqueue tests, jmg@ pointed out that these files were
originally space-based, not tab-spaced, and so the correction should
have been to get rid of the tabs that had been introduced in previous
changes, not the spaces. This change does that. This is a whitespace
only change; no functional change is intended.

Reported by: jmg@
MFC after: 3 days
Sponsored by: Dell EMC Isilon

Move tests/sys/kqueue/... to tests/sys/kqueue/libkqueue/...

This is being done to clearly distinguish the libkqueue tests
from the (soon to be imported) NetBSD tests.

MFC after: 58 days
Sponsored by: EMC / Isilon Storage Division