subs: Factor lnd_insque() out of lnd_sel(), ask_olist(), ...

subs: Factor lnd_insque() out of lnd_sel(), ask_olist(), ...

... get_dlist(), att_reacting_units().

This loses malloc() error checking in ask_olist() and get_dlist().  No
great loss, because we don't check in so many other places, including
att_reacting_units().  We should use a wrapper that terminates on
error, though.  Left for another day.

ask_olist(), get_dlist() and att_reacting_units() zero the struct
ulist with memset().  lnd_insque() doesn't, so these functions need to
zero any members not otherwise initialized explicitly now.

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