Make the source tile limit on spy actions a hard action enabler requirement

Move the requirement that the actor must be able to exist on its tile (the
source tile) to do a spy action from a test in is_diplomat_action_available() to
a requirement that always is appended to the actor requirements of the action
enablers of spy actions.

See patch #4995

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25755 a0f10bec-cc02-0410-94fc-a9cfff90b4cd