beep: Capsicumize

While it's unlikely that beep(1) will be used in a context where a
sandbox is particularly valuable, it does offer a simple demonstration
of entering a capability sandbox after opening required fds, and is
trivial to do.

Reviewed by: christos
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50709

(cherry picked from commit 54eda43cc1e6b1d73804b6c397a15002e3519555)