bsdinstall: Improve pkgbase handling for jails

Add a new --jail option to the pkgbase script which installs
jail-specific set variants if they exist:

• "minimal" is replaced with "minimal-jail"

• A kernel is not installed.

• For sets shown in the component selection dialogue, only show the appropriate variant (jail or non-jail) for the target.

Modify the jail script to pass --jail to the pkgbase script.

Remove the redundant --no-kernel option, which was added in 15.0 and
was only used to install jails.

MFC after: 3000ms
Reviewed by: ifreund_freebsdfoundation.org
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52829

(cherry picked from commit 8d0a90512ee35064697fbeffd0886eba4c82eb8d)