courier: Use DNSError.IsNotFound to identify NXDOMAIN

When resolving MX records, we need to distinguish between "no such
domain" and other kinds of errors. Before Go 1.13, this was not
possible, so we had a workaround that assumed any permanent error was a
"no such domain", which is not great, but functional.

Now that our minimum supported version is Go 1.15, we can remove the
workaround.

This patch replaces the workaround with proper logic using
DNSError.IsNotFound to identify NXDOMAIN results when resolving MX
records.

This requires to adjust a few tests, that used to work on environments
where resolving unknown domains (used for testing) returned a permanent
error, and now they no longer do so. Instead of relying on this
environmental property, we make the affected tests use our own DNS
server, which should make them more hermetic and reproducible.

test/t-02-exim/run.sh

@@ -38,6 +38,12 @@ fi
 mkdir -p .exim4
 EXIMDIR="$PWD/.exim4" envsubst < config/exim4.in > .exim4/config
 
+# Build with the DNS override, so we can fake DNS records.
+export GOTAGS="dnsoverride"
+
+# Launch minidns in the background using our configuration.
+minidns_bg --addr=":9053" -zones=zones >> .minidns.log 2>&1
+
 generate_certs_for srv-chasquid
 add_user user@srv-chasquid secretpassword
 add_user someone@srv-chasquid secretpassword
@@ -47,9 +53,11 @@ add_user someone@srv-chasquid secretpassword
 # Bypass MX lookup, so it can find srv-exim (via our host alias).
 mkdir -p .logs
 chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config \
+	--testing__dns_addr=127.0.0.1:9053 \
 	--testing__outgoing_smtp_port=2025 &
 
 wait_until_ready 1025
+wait_until_ready 9053
 
 # Launch exim at port 2025
 .exim4/exim4 -bd -d -C "$PWD/.exim4/config" > .exim4/log 2>&1 &