chroot: slightly cleanup

Highlights:

• Pull resolve_user() and resolve_group() out to make the main flow a bit easier to read
• Fix some edge-cases in user/group resolution: you can have fully numeric usernames, and they may or may not live within the valid ID range. Switch to just trying to resolve every specified group/user as a name, first, with a fallback to converting it to a numeric type and trying to resolve it as an ID.
• Constify locals in main() that don't need to be mutable, re-sort

Reviewed by: emaste, olce
Differential Revision: https://reviews.freebsd.org/D51509