client: pass the client struct to processCommand()

Start exporting the client struct as an opaque struct.  For now, pass
it only to processCommand() and processListOfCommands(), and provide a
function to extract the socket handle.  Later, we will propagate the
pointer to all command implementations, and of course to
client_print() etc.

src/client.h

@@ -21,6 +21,8 @@
 
 #include "os_compat.h"
 
+struct client;
+
 void client_manager_init(void);
 void client_manager_deinit(void);
 int client_manager_io(void);
@@ -28,6 +30,13 @@ void client_manager_expire(void);
 
 void client_new(int fd, const struct sockaddr *addr);
 
+/**
+ * Return the file descriptor of this client's socket.  This function
+ * will be removed once we have migrated to passing the client struct
+ * everywhere.
+ */
+int client_get_fd(struct client *client);
+
 int client_print(int fd, const char *buffer, size_t len);
 
 #endif