start: only use host's /dev/null when absolutely necessary
start: only use host's /dev/null when absolutely necessary

See comments for details, but basically, only use the host's /dev/null when
absolutely necessary (i.e. there is no reasonable /dev/null in the
container).

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

terceiro/lxc,QingweiPeterLan/cs188-lxc,QingweiPeterLan/cs188-lxc,ss1h2a3tw/lxc,cjwatson/lxc,caglar10ur/lxc,hfuCN/lxc,akshaykarle/lxc,LynxChaus/lxc,federicobriata/lxc,QingweiPeterLan/cs188-lxc,GreatFruitOmsk/lxc,akshaykarle/lxc,cjwatson/lxc,hfuCN/lxc,terceiro/lxc,GreatFruitOmsk/lxc,LynxChaus/lxc,caglar10ur/lxc,LynxChaus/lxc,aeris/lxc,federicobriata/lxc,ysbnim/lxc,terceiro/lxc,federicobriata/lxc,ss1h2a3tw/lxc,ysbnim/lxc,thmo/lxc,thmo/lxc,aeris/lxc,akshaykarle/lxc,ysbnim/lxc,hallyn/lxc,aeris/lxc,caglar10ur/lxc,cjwatson/lxc,hallyn/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,thmo/lxc,hallyn/lxc,ss1h2a3tw/lxc