packages: Add an "optional" set

This is everything in "base" but without compilers. This means we
have sets to support four basic workloads:

• "minimal" for a small installation where the user intends to add other packages by hand.

• "optional" for a complete installation on a production system which does not need to compile software.

• "devel", which can be added to either minimal or optional when compilers are required.

• "base" for a complete installation, including compilers, for users who don't want to interact with pkgbase and just want the complete system installed like it was before.

This is probably the last set want to add; any further metapackages
would be better treated as "task" packages intended to target one
specific workload.

MFC after: 3 days
Reviewed by: cperciva, bapt
Differential Revision: https://reviews.freebsd.org/D52777

(cherry picked from commit eaecc9551ae4d342fee921fb4c0a1eeb6bf68b86)