client: added client_write() and client_puts()

client_write() writes a buffer to the client and buffers it if
required.  client_puts() does the same for a C string.  The next patch
will add more tools which will replace fdprintf() later.

src/client.h

@@ -39,6 +39,16 @@ int client_get_fd(struct client *client);
 
 int client_is_expired(const struct client *client);
 
+/**
+ * Write a block of data to the client.
+ */
+void client_write(struct client *client, const char *data, size_t length);
+
+/**
+ * Write a C string to the client.
+ */
+void client_puts(struct client *client, const char *s);
+
 int client_print(int fd, const char *buffer, size_t len);
 
 #endif