dirmngr: Do not assume that /etc/hosts exists.

* dirmngr/dns-stuff.c (libdns_init): Do not bail out.

A standard Windows installation does not have a hosts file and thus we
can't bail out here. We should also not bail out on a Unix system
because /etc/hosts is just one method in nsswitch.conf.

• Fixes-commit: 88f1505f0613894d5544290a170119eb538921e5
• Signed-off-by: Werner Koch <wk@gnupg.org>