mail/milter-manager: Update to 2.1.6

• Fix plist error when either DOCS or EXAMPLES option is off
• Pet portclippy
• Tidy up Makefile with portfmt

ChangeLog: https://github.com/milter-manager/milter-manager/blob/2.1.6/NEWS
Reported by: Aoba Dai, Hideo Kuwabata, Naito Yuichiro (in freebsd-users-jp ML)
Tested by: Aoba Dai