mail/opendmarc: Add libspf2 as a run-time (library) dependency

libsfp2 is a build and run-time dependency for opendmarc.

Currently libspf2 is declared a BUILD_DEPENDS, which results in the
libspf2 package being removed by portmaster or pkg as an unecessary
package when using -s argument or autoremove command respectively.

This change correctly declares libspf2 as a LIB_DEPENDS, resulting
in both build and run-time dependency registration.

While I'm here:

• Convert MSPF conditional to an option helper
• Group and sort USE{S} entries
• Whitespace align OPTIONS
• Tweak SPF_DESC to match ./configure output for the option
• Sort pkg-plist

PR: 204050
Submitted by: Douglas <douglas douglasthrift net>
Approved by: portmgr (blanket)
MFH: 2015Q4