Upgrade databases/galera from 25.3.5_7 to 25.3.20_2

• Fetch from GitHub
• Adopt by devel@galeracluster.com
• Remove all documentation building from this port. The documentation is now online at http://galeracluster.com/documentation-webpages/
• Fix the linking with libboost_program_options if a static linkage has been chosen - it should be libboost_program_options.a, not libboost_program_options.so
• The new version of the port could have been 25.3.20. The reason for 25.3.20_2 is to ease upgrades for people that are using https://github.com/codership/freebsd-ports/tree/master/databases/galera which is at 25.3.20_1.

Relnotes: http://releases.galeracluster.com/release-notes-galera-25.3.20.txt