+ Used by server to select the appropriate parts of the
+ rest of the configuration. Ignored by the client.
+
+ secret
+ Looked up in the usual way, but used by client and server to
+ determine which possible peerings to try to set up, and which to
+ ignore.
+
+ We define the sets of putative clients and servers, as follows:
+ all those, for which there is any section (even an empty one)
+ whose name is based on <client> or <servername> (as applicable).
+ (LIMIT sections do not count.)
+
+ The server queue packets for, and accept requests from, each
+ putative client for which the config search yields a secret.