Implement dns_test_difffromchanges(), a function which enables preparing
a dns_diff_t structure from a mostly-textual representation of zone
database changes to be applied. This will improve readability of test
case definitions by allowing contents of a dns_diff_t structure, passed
e.g. to update_sigs(), to be represented in a human-friendly manner.
(cherry picked from commit 3c22af0d35)
(cherry picked from commit d4c603eb8a)
(cherry picked from commit 1f454b8d91)
(cherry picked from commit 7193496b6e)
f7a84380dd