BUG/MINOR: resolvers: default resolvers fails when network not configured

BUG/MINOR: resolvers: default resolvers fails when network not configured

Bug #1740 was opened again, this time a user is complaining about the
"can't create socket for nameserver". This can happen if the resolv.conf
file contains a class of address which was not configured on the
machine, for example IPv6.

The fix does the same as b10b1196b ("MINOR: resolvers: shut the warning
when "default" resolvers is implicit"), and uses the
"resolvers->conf.implicit" variable to emit the error.

Though it is not needed to convert the explicit behavior with a
ERR_WARN, because this is supposed to be an unrecoverable error, unlike
the connect().

Should fix issue #1740.

Must be backported were b10b1196b was backported. (as far as 2.6)

(cherry picked from commit 0d2ebb53f752502137479c9e753a96046d3254ab)
Signed-off-by: Willy Tarreau <w@1wt.eu>