config file checks and sample config

Changed the way the existence of config files is checked. It may be a
bit slower than before when there are many config files but maybe it
avoids other trouble.
Updated the sample config.

nsupdate.d/sample.config.dist

@@ -5,9 +5,16 @@ IP_CHECK_SITE="https://ip.dblx.io"
 
 # use drill instead of nslookup for hostname lookup
 USE_DRILL="YES"
+
+# Use IPv6 connection
 IPV6="NO"
+
+# Update an MX record
 MX="NO"
 
+# Suppress all messages
+SILENT=NO
+
 # Login credentials for the inwx admin interface
 INWX_USER="USERNAME"
 INWX_PASS='PASSWORD'