bsdinstall: Tweak pkgbase/dist set labels

Have the button labels refer to the artifact type directly (distribution
sets / packages), and use "Tech Preview" as packaged base is no longer
experimental.

Reviewed by: ivy, cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52999

(cherry picked from commit abd9424590ba37ac10e92723ad6428f0448024c1)