Renamed "city_style" entry in nation rulesets as "style".

See patch #4611


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24707 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Never call client_exit() directly inside gtk main loop, but exit the loop
so that end of ui_main() gets executed too.

See bug #21806


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24708 a0f10bec-cc02-0410-94fc-a9cfff90b4cd