dirmngr: Re-init libdns resolver on towel change of resolv.conf

* dirmngr/dns-stuff.c: Include sys/stat.h.
(RESOLV_CONF_NAME): New macro to replace a string.
(resolv_conf_changed_p): New.
(libdns_init): Call new function
(libdns_res_open): Ditto.

Don't panic. This is a simple change
to avoid complicated if-up.d hooks to reload resolv.conf.

• Suggested-by: Stefan Bühler <stbuehler@web.de>
• Signed-off-by: Werner Koch <wk@gnupg.org>