packages: Mark all sets as vital

At least "minimal" must be vital, because it's required for a functional
system. For example, we might create new packages in the future which
are required for the multi-user system and are part of the minimal set,
so it's important that users don't remove set-minimal to ensure they get
those new packages.

For the other sets, it's debatable whether they should be vital or not,
but let's start out with the assumption that if the user installed a
set, they should have to have to explicit request it be removed. This
means if, for example, they install set-devel, then remove lldb, pkg
won't remove set-devel and cause the next "pkg autoremove" to remove
everything else that was in set-devel.

We might want to adjust this later (or possibly adjust the behaviour
of pkg(8) itself) based on user feedback.

While here, fix a typo in the description of the devel set.

Suggested by: bapt
MFC after: 3 seconds
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D52562