mail/davmail: Update 6.1.0 => 6.5.1

Release Notes:
https://sourceforge.net/p/davmail/code/HEAD/tree/tags/6.5.1/RELEASE-NOTES.md

Improve port:

• Reordered Makefile variables per portclippy.
• Parametrize davmail with PORTNAME.
• Replace PORTVERSION with DISTVERSION.

PR: 293568
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>