Stop trying to clean up addresses, routes and "-L" file

Stop trying to clean up addresses, routes and "-L" file
whenever dhclient dies. Eliminates differences in handling
and thus need to intercept signals INT, TERM, USR1,
USR2. Eliminates need for 'zapzombies' field and thus
entire struct imsg_flushroutes. Eliminates need for 'imsg'
parameter to and associated logic in priv_flush_routes().

Address, routes and '-L' file are still cleaned out when
binding a lease.