Allocate metaserver scan mutex only when really needed.

See patch #4615


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24702 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Any unit stack where is a unit with Zoc is considered to have Zoc, even if the
first unit in the stack has not.

See bug #21782


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24704 a0f10bec-cc02-0410-94fc-a9cfff90b4cd