Honor check_all_prereqs at release time

check_all_prereqs is documented as controlling whether prereqs
were checked, but the before_release method was always including
them.

This commit fixes that oversight.
rearrange before_release to match before/after_build
add private testing function to reset %already_checked
add test for release prompt with/without check_all_prereqs
Merge pull request #1 from dagolden/honor-check-prereqs

Honor check_all_prereqs at release time