check_by_ssh no longer returns UNKNOWN if ssh(1) returns data on stderr.
But it can be enforced again by the new "--unknown-on-stderr" option.

---

The default logic of check_by_ssh results in an UNKNOWN state if the
ssh(1) process produces output on stderr. Using the "--skip-stderr=[n]"
option allows ignoring a certain amount of lines or disabling this check
altogether. Furthermore, passing the "--warn-on-stderr" option reduces
the exit code to WARNING.

The "--help" output does not document this behavior, only states that
"--warn-on-stderr" will result in the WARNING, but does not mention the
UNKNOWN by default.

The man page of ssh(1) mentions that debug information is logged to
stderr. This conflicts with the described logic, resulting in
check_by_ssh to go UNKNOWN, unless additional options are set.

Starting with OpenSSH version 10.1, ssh(1) will report warnings to
stderr if the opposite server does not support post-quantum
cryptography, <https://www.openssh.com/pq.html>.

This change, slowly being rolled out throughout the next months/years,
might result in mass-breakages of check_by_ssh.

By introducing a new "--unknown-on-stderr" option, enforcing the prior
default logic of an UNKNOWN state for data on stderr, and ignoring
output on stderr by default, check_by_ssh will continue to work. One
might even argue that this change converges actual implementation and
the documented behavior, as argued above.

---

$ ssh example '/usr/lib/nagios/plugins/check_dummy 0 demo'
** WARNING: connection is not using a post-quantum key exchange algorithm.
** This session may be vulnerable to "store now, decrypt later" attacks.
** The server may need to be upgraded. See https://openssh.com/pq.html
OK: demo

$ echo $?
0

$ ./check_by_ssh -H example -C '/usr/lib/nagios/plugins/check_dummy 0 demo'
OK: demo

$ echo $?
0

$ ./check_by_ssh -H example -C '/usr/lib/nagios/plugins/check_dummy 0 demo' --warn-on-stderr
Remote command execution failed: ** WARNING: connection is not using a post-quantum key exchange algorithm.

$ echo $?
1

$ ./check_by_ssh -H example -C '/usr/lib/nagios/plugins/check_dummy 0 demo' --unknown-on-stderr
Remote command execution failed: ** WARNING: connection is not using a post-quantum key exchange algorithm.

$ echo $?
3

---

Fixes monitoring-plugins#2147.