Set extra cause list pointers to NULL when lists are destroyed, to
fix a crash when the already freed extras were accessed after
disconnecting from server.

Reported by Jacob Nevins <jtn>

See bug #21318


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23822 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Fixed configure checks for compiling lua from our source tree to
be done when we're about to compile it, and not when we're going to
use lua from system.

See bug #21325


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23823 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Draw darkness sprites after all the terrain layers, not after first one.

Reported by Anonymous

See bug #21312


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23824 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added separate button for saving ruleset to freeciv-ruledit.

See patch #4322


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23827 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Removed enum extra_type_id.

See patch #4323


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@23828 a0f10bec-cc02-0410-94fc-a9cfff90b4cd