Fix nchan regex to prevent intercepting /sub/ in file paths

The regex pattern '~ /sub/(.*)$' matched /sub/ anywhere in the path,
including file paths like /mnt/disk2/test/foo/sub/file.iso. This caused
nginx to route these requests to the nchan subscriber endpoint instead
of serving the files.

Changed to '~ ^/sub/(.*)$' to only match paths that start with /sub/,
which is the correct behavior for the nchan websocket endpoint.

Fixes #2493

Author: mgutt

etc/rc.d/rc.nginx

@@ -412,7 +412,7 @@ build_locations(){
 	#
 	# nchan subscriber endpoint
 	#
-	location ~ /sub/(.*)$ {
+	location ~ ^/sub/(.*)$ {
 	    nchan_subscriber;
 	    nchan_subscriber_timeout 0;
 	    # nchan_authorize_request <url here>