• Add exec hook "exec.created". This is called when the jail is created and before exec.start is called. [1]
• Bump __FreeBSD_version.

This allows to attach ZFS datasets and various other things to be
done before any command/service/rc-script is started in the new
jail.

PR: 228066 [1]
Reviewed by: jamie [1]
Submitted by: Stefan Grönke <stefan@gronke.net> [1]
Differential Revision: https://reviews.freebsd.org/D15330 [1]