Prepare for implementing action probability calculation for each action

Probabilities other than impossible, not known and not implemented will
require action specific calculations. Clean up common/actions.c so it is
ready for them.

See patch #4525

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24545 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Add meta knowledge for unit class and unit class flag

See patch #4524

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24546 a0f10bec-cc02-0410-94fc-a9cfff90b4cd