umtxvar.h: Add missing include

Necessary to have the definition of 'struct timespec'.

No functional change.

Approved by: markj (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

sys: Remove $FreeBSD$: two-line .h pattern

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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

umtx: Add new pi_futex type.

Differential Revision: https://reviews.freebsd.org/D31250
MFC after: 2 weeks

umtx: Split do_unlock_pi on two counterparts.

The umtx_pi_frop() will be used by Linux emulation layer.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31238
MFC after: 2 weeks

umtx: Expose some of the pi umtx structures and API to the rest of the kernel.

Differential Revision: https://reviews.freebsd.org/D31237
MFC after: 2 weeks

umtx: Add umtxq_requeue Linux emulation layer extension.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31235
MFC after: 2 weeks

umtx: Add bitset conditional wakeup functionality.

The bitset is a Linux emulation layer extension. This 32-bit mask, in which at
least one bit must be set, is used to select which threads should be woken up.

The bitset is stored in the umtx_q structure, which is used to enqueue the waiter
into the umtx waitqueue. Put the bitset into the hole, that appeared on LP64 due
to data alignment, to prevent the growth of the struct umtx_q.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31234
MFC after: 2 weeks

umtx: Expose some of the umtx structures and API to the rest of the kernel.

Differential Revision: https://reviews.freebsd.org/D31233
MFC after: 2 weeks

umtx: Expose struct abs_timeout to the rest of the kernel.

Add umtx_ prefix to all abs_timeout facility and add declaration for it.
For consistency with others abs_timeout mark inline abs_timeout_init2.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31249
MFC after: 2 weeks

umtx: Split umtx.h on two counterparts.

To prevent umtx.h polluting by future changes split it on two headers:
umtx.h - ABI header for userspace;
umtxvar.h - the kernel staff.

While here fix umtx_key_match style.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31248
MFC after: 2 weeks