Use msync rather than fsync to flush the entire disc

This involves storing the size of the mapped disc in the client struct,
and then supplying that to the msync command.

src/server/client.h

@@ -3,6 +3,7 @@
 
 #include <signal.h>
 #include <time.h>
+#include <inttypes.h>
 
 /** CLIENT_HANDLER_TIMEOUT
  * This is the length of time (in seconds) any request can be outstanding for.
@@ -31,6 +32,8 @@ struct client {
 	int     fileno;
 	char*   mapped;
 
+	uint64_t  mapped_size;
+
 	struct self_pipe * stop_signal;
 
 	struct server* serve; /* FIXME: remove above duplication */