pkgbase: make only vital packages vital, not their sub-packages

make "vital" a replaceable, which defaults to "false" and only set it for
the main clib, utilities and runtime packages, not their sub-packages

PR: 254174
Differential Revision: https://reviews.freebsd.org/D29224

(cherry picked from commit 1c1ff7979571bf07c05a48e857b7b285b037410f)