fix: wire up exec, fw auth vars, and prune dead detection keys

- Template now emits `exec=` so ddclient_exec actually toggles dry-run (was documented and defaulted but never written to the config).
- Rename fw_login/fw_password defaults to ddclient_fw_login/ddclient_fw_password to match what the template reads; previous names were silently dead.
- Drop unused address/interface/command/skip keys from the ddclient_ipv{4,6}_detection defaults — the template only reads method/source/skip_pattern. Fix the `# Method:` comments to list the v4/v6-suffixed names actually in use.

templates/ddclient.conf.j2

@@ -48,6 +48,7 @@ debug={{ ddclient_debug | ternary('yes', 'no') }}
 # ============================================================
 # Update Behavior
 # ============================================================
+exec={{ ddclient_exec | ternary('yes', 'no') }}
 retry={{ ddclient_retry | ternary('yes', 'no') }}
 force={{ ddclient_force | ternary('yes', 'no') }}
 {% if ddclient_postscript %}