Make the DNS over HTTPS support optional

This commit adds two new autoconf options `--enable-doh` (enabled by
default) and `--with-libnghttp2` (mandatory when DoH is enabled).

When DoH support is disabled the library is not linked-in and support
for http(s) protocol is disabled in the netmgr, named and dig.

bin/tests/system/checkconf/tests.sh

@@ -75,6 +75,12 @@ do
 	n=`expr $n + 1`
 	echo_i "checking that named-checkconf detects no error in $good ($n)"
 	ret=0
+	if ! $FEATURETEST --with-libnghttp2
+	then
+		case $good in
+		good-doh-*.conf) continue;
+		esac
+	fi
 	$CHECKCONF $good > checkconf.out$n 2>&1
 	if [ $? != 0 ]; then echo_i "failed"; ret=1; fi
 	status=`expr $status + $ret`