transport: Don't hardcode material weights

transport: Don't hardcode material weights

tran_plane() computes a plane's weight from its materials.  It
hardcodes lcm weight 1, hcm weight 2, and military weight 0.  Use
ichr[].i_lbs instead, which is 1 in the stock game for all three
materials.

While there, support arbitrary materials, even though they aren't yet
possible, just to avoid unnecessary assumptions on possible build
materials.

Since the stock game's planes use fewer military than hcms, they
become up to 15% lighter, except for zep, which becomes 10% heavier.
Missiles use no military and become 20-33% lighter.

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