power: Use ship, plane, land unit tech instead of nation's

power: Use ship, plane, land unit tech instead of nation's

Actual abilities of ships, planes and land units depend almost
completely on the individual unit's tech, not the nation's tech.  The
power factor should reflect that.

The power value of a unit is of the form

    base value * (20 + nation's tech level) / 500

Change it to

    base value * (20 + unit's tech level) / 500

Note that a plane's base value still depends on the nation's tech
level.  This commit merely makes the absurdity stand out a bit more.
To be fixed later.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>