plugin/{file,auto}: drop fsnotify (#1090)

* plugin/{file,auto}: drop fsnotify

Reload every minute. This is more deterministic then fsnotify. Also
other thing cropped up: sharing zone files between zone; there is only
1 fsnotify event and we need to fan out the reload to all zone files.
This is a large rewrite (which could still be done), for now, poll the
zone file on disk.

Give serial no change a special error type so we can check for this.
Improve the logging for reloading:

2017/09/19 07:34:39 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263060
2017/09/19 07:34:45 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263059
2017/09/19 07:34:51 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263060

Fixes #1013

* typo

plugin/auto/README.md

@@ -28,8 +28,8 @@ are used.
   name `db.example.com`, the extracted origin will be `example.com`. **TIMEOUT** specifies how often
   CoreDNS should scan the directory, the default is every 60 seconds. This value is in seconds.
   The minimum value is 1 second.
-* `no_reload` by default CoreDNS will reload a zone from disk whenever it detects a change to the
-  file. This option disables that behavior.
+* `no_reload` by default CoreDNS will try to reload a zone every minute and reloads if the
+  SOA's serial has changed. This option disables that behavior.
 * `upstream` defines upstream resolvers to be used resolve external names found (think CNAMEs)
   pointing to external names. **ADDRESS** can be an IP address, and IP:port or a string pointing to
   a file that is structured as /etc/resolv.conf.