bsdinstall: Only offer to enable services which are installed

With pkgbase, users may decide to install a minimal set of packages
that's missing some daemons. Check which services are installed, and
only include the ones which are present in the dialogue.

MFC after: 1 day
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D52646

(cherry picked from commit 2507698b630db3d88b0aab12c348873f827255a2)