Log Message:
mpop: update to 1.4.21. Changes:

- Build with libgsasl by default again (if found).
- Add IDN support to --configure (build with libidn2 to enable this).
- Add support for SCRAM-SHA-256-PLUS and SCRAM-SHA-1-PLUS
  authentication, and prefer SCRAM methods over the PLAIN method because
  of their superior properties.

Packaging adjusted to more closely resemble msmtp. Build-tested on
NetBSD and macOS.