In particular, don't report EAGAIN on send() calls with no user
data, which might trigger a KASSERT in asyc IO.

Reported by:	syzbot+3b4dc5d1d63e9bd01eda@syzkaller.appspotmail.com
MFC after:	1 week

The main change was v1.57 by djm@:

  Randomise the rekey interval a little. Previously, the chacha20
  instance would be rekeyed every 1.6MB. This makes it happen at a
  random point somewhere in the 1-2MB range.

Reviewed by:	csprng (markm, cem)
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D36088

The new tests exercise simulated COW that occurs when the protections on
a wired, copy-on-write mapping are changed from read-only to read-write.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35636

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D36078

Here, the provider is responsible for updating the trapframe to redirect
control flow and for computing the return address.  Once software-saved
registers are restored, the emulation shifts the remaining context down
on the stack to make space for the return address, then copies the
address provided by the invop handler.  dtrace_invop() is modified to
allocate temporary storage space on the stack for use by the provider to
return the return address.

This is to support a new provider for amd64 which can instrument
arbitrary instructions, not just function entry and exit instructions as
FBT does.

In collaboration with:	christos
Sponsored by:	Google, Inc. (GSoC 2022)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

dtrace invop handlers have access to the whole trapframe, just use that
to extract %rax/%eax for return probes instead of relying on an
additional parameter to the handler.  No functional change intended.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation