ntpd: retry initial DNS resolution (forever, no timeout for now).

ntpd: retry initial DNS resolution (forever, no timeout for now).

Some users start ntpd on boot, and don't babysit it. If it dies because
DNS is not yet up and therefore NTP servers can't be found, users are
not happy.

Example behavior with a peer name which can't be resolved:

ntpd: bad address 'qwe.rty.ghj.kl'
...5 sec...
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
ntpd: bad address 'qwe.rty.ghj.kl'
...

Based on the patch by Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>

function                                             old     new   delta
resolve_peer_hostname                                  -      81     +81
ntpd_main                                           1130    1061     -69
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 81/-69)             Total: 12 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>