jail: add JID, JNAME and JPATH to environment for exec.* commands

[Defenso-QTH]

Although variable substitution is available in the jail configuration file, the jail identifier is often not since it is dynamically attributed at run time.

In order to facilitate scripting of exec.* commands executed on the system, this change sets the JID, JNAME and JPATH environment variables.

These variables are not added when using exec.clean. Neither are they for commands executed inside jails, to avoid disclosing information about the host system.

[Crest]

You can write this to export the jail id and jail name to the shell script, but the quoting gets ugly:

exec.start = "
    export JID='$jid' JNAME='$name'
    echo \"Hello \$JNAME from \$JID.\"
";

[Defenso-QTH]

That could work. It seems my issue comes from using iocage, which does not rely on jail to exec the prestart/poststart commands but does it itself, so we don't get replacement, and it does not set environment variables either.

https://github.com/freebsd/iocage/blob/c55e05aebb305b4013554ec80298113c50d1c74c/iocage_lib/ioc_start.py#L873C1-L877C14

        # Running exec_poststart now
        poststart_success, poststart_error = \
            iocage_lib.ioc_common.runscript(
                exec_poststart
            )

Maybe it makes more sense patching iocage then.