Initialise _res.nsaddr_list in res_init(), fixing some programs which depend

Initialise _res.nsaddr_list in res_init(), fixing some programs which depend
on bind resolver internals. Note, the list will not be updated if resolv.conf
is changed while the program runs unless the caller resets the RES_INIT flag.
Approach suggested by otto@, original diff from Kapetanakis Giannis, fix
suggested by tedu@ and dug up again by Riccardo Mottola.  Looks ok to eric@.