Use execve(2) instead of fexecve(2).

[crest_freebsd_rlwinm.de]

Summary

The dynamic jail.conf is probably a (shell) script.
In that case fexecve(2) will result in kernel running
the interpreter with the script as argument e.g. /dev/fd/$n.
This only works if /dev/fd was mounted with the non-default "nodup"
option, because the kernel doesn't honor O_RDONLY | O_EXEC
and instead opens the file (description) with just FEXEC.

See PR #294780

Event: Wiesbaden Hackathon 202604