NPCs: Make NPC automatic fire threshold halfway sensible

NPCs are only supposed to use automatic fire instead of single shots
against dangerous enemies, and to hold their fire against non-threatening
enemies.

However, the danger threshold for automatic fire is 0.5, which means
NPCs are using automatic fire against just about everything.

Raise the threshold to 15 (moderately dangerous enemies) and increase
the threshold for holding fire to 3.  Weaker enemies will be attacked in
melee if they're the highest priority target.

src/npcmove.cpp

@@ -1297,7 +1297,7 @@ npc_action npc::method_of_attack()
             return m.melee() || m.flags.count( "NPC_AVOID" ) ||
                    !m->ammo_sufficient( m.qty ) || !can_use( *m.target ) ||
                    m->get_gun_ups_drain() > ups_charges ||
-                   ( danger <= ( ( m.qty == 1 ) ? 0.0 : 0.5 ) && !emergency() ) ||
+                   ( ( danger <= ( m.qty == 1 ? 3.0 : 15.0 ) ) && !emergency() ) ||
                    ( rules.has_flag( ally_rule::use_silent ) && is_player_ally() &&
                      !m.target->is_silent() );