Introduce the UnitState requirement type with the test Transported

Add the new requirement type UnitState. UnitState is for checking the properties
of a unit's state. It is intended to be like CityTile, but for units. The first
test, Transported, checks if the unit is transported.

See patch #4983

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25732 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Changing the musicset option takes effect immediately and not only after client restart

Reported by mqtx

See bug #22390


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25733 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added AM_DIST_FORMATS variable, ignored by regular automake, to work toward
automake-ng support.

See patch #4975


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25734 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Some Coding Style corrections for sdl2-client.

See patch #4835


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25735 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added National Culture effects "National_Performance" and "National_History"

See patch #4950


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25736 a0f10bec-cc02-0410-94fc-a9cfff90b4cd