stable/12: jail(2) failures after ca9ab8ea1774

ElectroBSD contains patches for ggatec and ggated to
drop privileges using jail(2).

After rebasing from stable/11 to stable/12 this stopped working
for ggated but not for ggatec.

The problem seems to be caused by:

| commit ca9ab8ea17748a1758701fde262cb272fb757989
| Author: Jamie Gritton <jamie@FreeBSD.org>
| Date:   Fri Feb 19 14:13:35 2021 -0800
|
|     MFC jail: Change both root and working directories in jail_attach(2)
|
|     jail_attach(2) performs an internal chroot operation, leaving it up to
|     the calling process to assure the working directory is inside the jail.
|
|     Add a matching internal chdir operation to the jail's root.  Also
|     ignore kern.chroot_allow_open_directories, and always disallow the
|     operation if there are any directory descriptors open.
|
|    Reported by:    mjg
|    Approved by:    markj, kib

One of the differences between ggated and ggatec that is probably
relevant is that ggated uses pidfile_open().

Is jail(2) expected to work for applications using the
pidfile family of functions with pid files located outside
the jail directory?

Currently the struct returned by pidfile_open() contains
a file descriptor to the directory containing the pid file
which I suspect is the problem.

ElectroBSD's ggated tries to jail into /var/empty while
the pid file is located in /var/run.

Shouldn't pidfile_open()'s use of cap_rights_limit() make
this acceptable?

ElectroBSD's ggate[cd]-related patches are available at:
<https://www.fabiankeil.de/sourcecode/electrobsd/ElectroBSD-12-0b7b773f3ef4-2021.11.10-ggate.diff>
They should apply cleanly on stable/12 e644c87aa.

For testing purposes I added a -j option and a sysctl
to toggle the behaviour added in ca9ab8ea1774:

fk@t520 ~ $sudo ggated -v -j
info: Listen on port: 3080.
error: Unable to jail process in directory /var/empty
error: Exiting.
fk@t520 ~ $sudo sysctl kern.pwd_chroot_chdir_check_open_directories=0
kern.pwd_chroot_chdir_check_open_directories: 1 -> 0
fk@t520 ~ $sudo ggated -v -j
info: Listen on port: 3080.
debug: Privileges successfully dropped using jail+setgid+setuid.

Fabian