TLSRPT support (RFC 8460)

[wietse-postfix]

Hello, I'm the main author of the Postfix mail server. I am adding TLSRPT supprt (RFC 8460). To make that work with the MTA-STS plugin I extended the Postfix smtp_tls_policy_maps syntax with a number of attributes that are useful in TLSRPT reports, such as:

• MTA-STS policy domain name and policy strings,
• MX host patterns,

I also added:

• Optional failure attribute that forces a verification failure when a certificate would satisfy conventional PKI. "forced failure" is needed in DANE support ,but I can remove this is if MTA-STS does not need it.

I'd like to solicit feedback on https://www.postfix.org/TLSRPT_README.html; of particular interest are the sections on "MTA-STS Support" and "Limitations". Questions and suggestions are welcome.