Added data directory "override" containing empty files of the types needed for
creating custom nations. They are included from respective main files, and user can
simply place their own files with real nations earlier in the data path, overriding
the empty ones.

Requested by Jacob Nevins <jtn>

See patch #3926


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25112 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Handle metaserver reply to client in memory instead of writing it to temporary file.

See patch #4779


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25113 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Iterate over city styles only once when constructing sdl(2)-client nation selection dialog,
fixing a bug that first of the styles appeared twice.

See bug #22148


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25114 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added note about freeciv-ruledit to README.packaging list of changes in 2.6.

See patch #4780


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25115 a0f10bec-cc02-0410-94fc-a9cfff90b4cd