Simplify ask_olist and get_dlist() not to re-read land units

Simplify ask_olist and get_dlist() not to re-read land units

getland() is obviously redundant in get_dlist(), because the land unit
can't have changed since the previous read.

What's up with ask_olist() is less obvious.  For each capable land
unit, ask_olist() asks the player whether to attack with it.  If it
attacks, it gets copied into olist.  Since asking the player yields
the processor, these copies can become stale.  However, re-reading
fixes that just for the last one.  Moreover, the code copes with stale
copies just fine: ask_olist() is always directly followed by
att_get_offense() or att_move_in_off(), and both replace the
potentially stale copy in olist.