use unsynced rng for unit traits

this has 2 reasons:
1) In the 'unit::unit(const unit_type& u_type ..' ctor the facting is
usually overwritten after the unit is constructed (at least the normal
unit recruit code does that) so calling the synced rng there has no
other effect than blocking undoing (in case that the unit has no traits,
otherwise undoing would already be imposible because of trait
generation)

2) in the 'unit::unit(const config& cfg ...' ctor it might make sense to
use the synced rng, but since iirc the facing attribute is already
unsynced since it changed by the drawing code (which might be skipped if
animations are disabled) we also use the unsyced rng here to not block
undoing (in that rare case).

src/units/unit.cpp

@@ -437,7 +437,7 @@ unit::unit(const config& cfg, bool use_traits, const vconfig* vcfg)
 
 	random_traits_ = cfg["random_traits"].to_bool(true);
 	facing_ = map_location::parse_direction(cfg["facing"]);
-	if(facing_ == map_location::NDIRECTIONS) facing_ = static_cast<map_location::DIRECTION>(randomness::generator->get_random_int(0, map_location::NDIRECTIONS-1));
+	if(facing_ == map_location::NDIRECTIONS) facing_ = static_cast<map_location::DIRECTION>(randomness::rng::default_instance().get_random_int(0, map_location::NDIRECTIONS-1));
 
 	if(const config& mods = cfg.child("modifications")) {
 		modifications_ = mods;
@@ -668,7 +668,7 @@ unit::unit(const unit_type& u_type, int side, bool real_unit, unit_race::GENDER
 	, overlays_()
 	, role_()
 	, attacks_()
-	, facing_(static_cast<map_location::DIRECTION>(randomness::generator->get_random_int(0, map_location::NDIRECTIONS-1)))
+	, facing_(static_cast<map_location::DIRECTION>(randomness::rng::default_instance().get_random_int(0, map_location::NDIRECTIONS-1)))
 	, trait_names_()
 	, trait_descriptions_()
 	, unit_value_()