In extra structure, replaced tile_special_type, limited to hardcoded enum values, with generic
index in the EC_SPECIAL list. Removed all the specials related code outside savecompat.[ch]
that can be removed already.


See patch #4334


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23859 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Add a scenario option to restrict the set of nations players can pick to
those for which start positions have been defined in the scenario.
(This used to be the default behaviour in 2.0.x, but has not been since
then, probably by accident. The new default behaviour remains to not
restrict nation choice, and none of the supplied scenarios use the new
option.)
Also, improve comments and diagnostics about what happens in this case.

See gna bug #21268.

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23860 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Do not send S_LAST when should send extra index with no extra associated.

See bug #21341


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23862 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Run Qt modpack installer modpack installation functionality in separate thread so that
gui remains responsive.

See patch #4337


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23863 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Save terrain control, terrains, and resources of terrains.ruleset from freeciv-ruledit.

See patch #4336


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23865 a0f10bec-cc02-0410-94fc-a9cfff90b4cd