*: add "noexcept" to many, many function prototypes

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

src/command/NeighborCommands.hxx

@@ -30,7 +30,7 @@ class Response;
 
 gcc_pure
 bool
-neighbor_commands_available(const Instance &instance);
+neighbor_commands_available(const Instance &instance) noexcept;
 
 CommandResult
 handle_listneighbors(Client &client, Request request, Response &response);