Add DNSSEC signing, key backups and Porkbun support

Zones get a DNSSEC setting: named signs them with BIND's default (or a
custom) dnssec-policy and inline signing, with keys in a per-view key
directory. Jarl shows the keys, the DS record and when it is safe to
publish it, checks the parent's DS records (check-ds, which also tells
named), and refuses steps that would break a signed domain: switching
straight to unsigned, finishing a turn-off too early, deleting or moving
a signed zone, TTLs above max-zone-ttl. Turning off goes through the
insecure policy; wipe starts over once the parent has no DS.

Backups store the DNSSEC keys and restore puts them back for named; the
backup unit gets CAP_DAC_READ_SEARCH to read them. Jarl raises a zone's
serial above named's whenever it writes an older one (after a restore).
The serials check compares the signed serial.

With a Porkbun API key in jarl.yaml, the registrar's name servers, glue
records and DS records can be shown and changed, each change checked
first (dry run, force to override).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dnssec/testdata/Kexample.test.+013+22642.key

@@ -0,0 +1,6 @@
+; This is a key-signing key, keyid 22642, for example.test.
+; Created: 20261008190331 (Thu Oct  8 19:03:31 2026)
+; Publish: 20261008190331 (Thu Oct  8 19:03:31 2026)
+; Activate: 20261008190331 (Thu Oct  8 19:03:31 2026)
+; SyncPublish: 20261009190831 (Fri Oct  9 19:08:31 2026)
+example.test. 3600 IN DNSKEY 257 3 13 fiSDgJJGHyAp36ZNGOeb45gn+O1pF3hCbTUPHeJ3ztIuisGoywmo7Be3 +vUPyC6e9BmTCIRIeElB4f2o9HoPXg==