libdns: Sync to upstream.

* dirmngr/dns.c (dns_nssconf_loadfile): Handle exclamation mark.

Cherry pick from master commit:

3e6ad302eaf3a4a9f3e60379133b3dfdbe0e1b2d

Reverting local change, merge upstream's debug-tracing branch.
(commit 21281fc1b63bb74d51762b8e363c49b1a258783d)

• Fixes-commit: d4c0187dd93163f12e9f953366adef81ecf526a6
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>