AI: Use the spy action Sabotage Unit if Bribe Unit was impossible.

See patch #4153

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24715 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Action probability for the Bribe Unit action

Don't include information about the bribe cost in the probability. This is so a
player can learn the odds of success before deciding to get the required amount.

See patch #4628

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24716 a0f10bec-cc02-0410-94fc-a9cfff90b4cd