fix: standardize X-Mailer, version string, and forensic relay tracing

[thegushi]

Summary

• The milter's AFRF failure-report path used User-Agent: %s/%s; opendmarc-reports already used X-Mailer: opendmarc-reports v<version> for aggregate reports. Both now use X-Mailer, the established email convention for identifying generating software.
• opendmarc-reports' $version used the plain release version with no git-describe logic, while the milter's DMARCF_VERSION already runs git describe --tags --dirty at configure time. Identical dev builds reported different version strings depending on which program authored the report. configure.ac now AC_SUBSTs DMARCF_VERSION for use in opendmarc-reports.in.
• opendmarc-reports --forensic relays the milter-composed message via Net::SMTP directly to the configured server, completely untouched -- unlike the direct sendmail -t path, which picks up a normal Received: header for free from the local MTA. There was previously no way to tell, from the message itself, whether a forensic report went out via the milter directly or was relayed through opendmarc-reports. A single Received: by <host> (opendmarc-reports v<version>) via forensic relay; <date> line is now prepended before sending, added before the message is archived via --archive-dir so the archived copy matches what was actually sent, and applied identically regardless of --verp (VERP only rewrites the SMTP envelope sender per recipient; it never touches message content, so this doesn't interact with it).

Test plan

• Full make + make check: 0 warnings, 0 errors, all 11 test programs pass
• Confirmed build-config.h's DMARCF_VERSION and the generated reports/opendmarc-reports's $version now resolve to the identical git-describe string on a dirty dev build
• perl -c reports/opendmarc-reports: syntax OK
• Manually verified the Received: line renders correctly ahead of a sample forensic message