config: Make plains build and mobility cost match wilderness

config: Make plains build and mobility cost match wilderness

Wilderness costs nothing to build (since Chainsaw 3), but efficiency
doesn't affect their mobility cost of 0.4 (commit 083003a, v4.3.6).

Plains (since 4.2.0) cost the customary $100, and mobility cost drops
from 0.4 at 0% to 0.2 at 100% efficiency.

Change plains to match wilderness: waive build cost, set mob1 to 0.4

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