You can't steal a tech from someone that don't have any techs you don't have

Take into account if the target player has any techs that can be stolen when
calculating the action probability of the spy actions steal tech and
targeted steal tech.

Idea stolen from Freeciv-web.

See patch #4539

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24569 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Players can't meet if one of them has activated the No_Diplomacy effect.

Add a check to the could_meet_with_player() function. It is used to activate
the button that let you have a diplomatic meeting with another player in the
user interface. This prevents the player from being tricked into thinking
that he can meet the other player.

See bug #21714

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24570 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added requirement type "Style"

See patch #4533


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24572 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Do not assume return value of can_unit_attack_tile() is unit_attack_result.
Renamed all those can_unit_attack_...() functions that return unit_attack_result
instead of boolean as unit_attack_..._result().

See bug #21728


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24573 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Make action target kind names better fit to appear in generated text.

Currently the names are only used in an assertion message when the wrong
target kind is used in is_action_enabled_unit_on_*() and action_prob_vs_*()
The generated text becomes a lot better in those cases.

See patch #4546

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24576 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Add the field ui_name to actions. Don't use it yet.

See patch #4547

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24577 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
AI does not prefer luxury over science when player is on war footing.

Patch by David Fernandez <bardo>

See bug #21640


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24578 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Mention what actions an unit can do in its auto generated help text.

This doesn't replace the descriptions of what a spy action does in the rule
set specified unit help text.

See patch #4548

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24580 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Action: Introduce actor kind

The only kind of actor in the current action enabler controlled actions is
unit. It is better to introduce actor kind now and check for it as code
where it matters is introduced than to hope all the relevant places are
found and the proper checks will be added when other kind of actors appear.

See patch #4549

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24581 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Add meta knowledge for the MaxUnitsOnTile requirement type

See patch #4552

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24582 a0f10bec-cc02-0410-94fc-a9cfff90b4cd