Add the ability to run executable jail.conf and read its stdout.

[crest_freebsd_rlwinm.de]

Summary

The redundancy between similar jails and some dynamic behavior can't
be captured by .include "<pattern>"; alone and still allow
jail -c <name> to start jails without a wrapper.

Take the same way out as automountd(8) and run executable configuration
files. These scripts are expected to write a valid configuration
(snippet) to standard output and return exit status 0.