Don't store land unit stats in struct lndstr, part 1

Don't store land unit stats in struct lndstr, part 1

New lnd_att(), lnd_def(), lnd_vul(), lnd_spd(), lnd_vis(), lnd_frg(),
lnd_acc(), lnd_dam(), lnd_aaf() replace the struct lndstr members with
the same names.

Make land unit selectors att, def, vul, spd, vis, frg, acc, dam, aaf
virtual.