SMTP: Adapt scripts for enable_rfc822_msg_file_analysis

Specifically, set a MIME part's parent_id to the rfc822_msg_fuid if it
is set and take into account the current rfc822_msg_fuid for describe_file()
to avoid fuid collisions of the top-level RFC822 message and the first
MIME part.

testing/btest/Baseline/coverage.record-fields/out.default

@@ -779,6 +779,7 @@ connection {
         * process_smtp_headers: bool, log=F, optional=T
         * rcptto: set[string], log=T, optional=T
         * reply_to: string, log=T, optional=T
+        * rfc822_msg_fuid: string, log=F, optional=T
         * second_received: string, log=T, optional=T
         * subject: string, log=T, optional=T
         * tls: bool, log=T, optional=T