Permit Helicopter embarking in civ2civ3 ruleset

Add embarks and disembarks vectors for all modern infantry types.
Allow Riflemen to upgrade to Marines to improve "modern" definition.

Patch by David Fernandez <bardo>

See bug #21952

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24799 a0f10bec-cc02-0410-94fc-a9cfff90b4cd