Migrate nreqs to reqs

* When reading a ruleset, store nreqs as reqs with reversed presence
* Remove all other engine support for nreqs

See patch #4679

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25203 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Add pythagorean penalty for diagonal moves

Disabled by default in all rulesets

Requested by GriffonSpade @ http://forum.freeciv.org/f/viewtopic.php?f=13&t=51

See patch #4819

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25204 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Worklist explanation improvements

* Explain why units can't be built (government, improvement, tech)
* Consider negated requirements when explaining improvement postponement
* Rework improvement purging logic: purge in more unachievable cases
* Change double-spacing in messages to single-spacing

See bug #21420

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25205 a0f10bec-cc02-0410-94fc-a9cfff90b4cd