dmarc: new DMARC plugin

uses dmarc_httpd from Mail::DMARC for processing and reporting
connection.js: save A-R header after data_post

so DKIM and DMARC plugins get a chance to update it
spf: store SPF results in ResultStore (was notes)
dmarc: save results in 'results' (was in a note)
dmarc: cleanup rebase litter
dmarc: use new boolean config option
dmarc: add intermediate header in cmd_data, and update

in data_post_respond if new results
dmarc: get DKIM validation from X-Spam-Tests
dmarc: wrap JSON parser in a try

since it may get garbage
dmarc: check for txn before saving results