There's a currently ad-hoc protocol to hand off the FreeBSD kernel
payload between the loader and the kernel itself when Xen is in the
middle of the picture. Such protocol wasn't very resilient to changes
to the loader itself, because it relied on moving metadata around to
package it using a certain layout. This has proven to be fragile, so
replace it with a more robust version.

The new protocol requires using a xen_header structure that will be
used to pass data between the FreeBSD loader and the FreeBSD kernel
when booting in dom0 mode. At the moment the only data conveyed is the
offset of the start of the module metadata relative to the start of the
module itself.

This is a slightly disruptive change since it also requires a change
to the kernel which is contained in this patch. In order to update
with this change the kernel must be updated before updating the
loader, as described in the handbook. Note this is only required when
booting a FreeBSD/Xen dom0. This change doesn't affect the normal
FreeBSD boot protocol.

This fixes booting FreeBSD/Xen in dom0 mode after
3630506b9daec9167a89bc4525638ea45a00769e.

Sponsored by:		Citrix Systems R&D
MFC after:		3 days
Reviewed by:		tsoome
Differential Revision:	https://reviews.freebsd.org/D28411

This follows select by eleminating the use of filedesc lock.
This is a win for single-threaded processes and a mixed bag for others
as at small concurrency it is faster to take the lock instead of
refing/unrefing each file descriptor.

Nonetheless, removal of shared lock usage is a step towards a
mtx-protected fd table.

Since most select users are single-threaded this avoid a lot of work
in the common case.

For example select of 16 fds (ops/s):
before:	2114536
after:	2991010

This can be used by single-threaded processes which don't share a file
descriptor table to access their file objects without having to
reference them.

For example select consumers tend to match the requirement and have
several file descriptors to inspect.

Update the reference of which file to update in the doc tree when
bumping __FreeBSD_version.

This fixes one of the sys/audit tests when running them on tmpfs.

Reviewed By:	delphij, kib
Differential Revision: https://reviews.freebsd.org/D28387

This tests fork()s, so if there is still data in the stdout buffer on fork
it will print it again in the child process. This was happening in the
CheriBSD CI and caused the test to complain about malformed TAP output.

Reviewed By:	ngie
Differential Revision: https://reviews.freebsd.org/D28397

Reviewed By:	lwhsu
Differential Revision: https://reviews.freebsd.org/D28396

Reported by:	jrtc27

It's possible when adding a jail that its dying parent comes back to
life.  Only allow that to happen when JAIL_DYING is specified.  And if
it does happen, call PR_METHOD_CREATE on it.

No need for "foo$(printf "\t")", $'\t' is both more readable and still
functional.

Reported-by:	Jamie Landeg-Jones <jamie@catflap.org>

vfs_cache_lookup() has already done the appropriate VEXEC check, therefore
we must not re-check in VOP_CACHEDLOOKUP.

This fixes O_SEARCH semantics on tmpfs and removes a redundant descent into
VOP_ACCESS() in the common case.

Reported-by:	arichardson (via CheriBSD Jenkins CI)
Reviewed-by:	kib
MFC-after:	3 days
Differential Revision:	https://reviews.freebsd.org/D28401