civ2civ3: base defense against all unit classes.
Make units inside bases less vulnerable against one type of attack,
especially at the final stages of the game, by giving fortresses a 50%
defense bonus against air unit and by giving Airbase a 50% defense bonus
against land and sea units.
Increase the vision radius inside airbases to the same value as fortresses
with astronomy.
Patch by David Fernandez <bardo>
See gna patch #8123
git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@35425 a0f10bec-cc02-0410-94fc-a9cfff90b4cd