client: added client_printf()

Based on client_puts(), client_printf() is the successor of
fdprintf().  As soon as all fdprintf() callers have been rewritten to
use client_printf(), we can remove fdprintf().

src/client.c

@@ -797,3 +797,32 @@ void client_puts(struct client *client, const char *s)
 {
 	client_write(client, s, strlen(s));
 }
+
+void client_vprintf(struct client *client, const char *fmt, va_list args)
+{
+	va_list tmp;
+	int length;
+	char *buffer;
+
+	va_copy(tmp, args);
+	length = vsnprintf(NULL, 0, fmt, tmp);
+	va_end(tmp);
+
+	if (length <= 0)
+		/* wtf.. */
+		return;
+
+	buffer = xmalloc(length + 1);
+	vsnprintf(buffer, length + 1, fmt, args);
+	client_write(client, buffer, length);
+	free(buffer);
+}
+
+mpd_fprintf void client_printf(struct client *client, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	client_vprintf(client, fmt, args);
+	va_end(args);
+}

src/client.h

@@ -20,6 +20,7 @@
 #define INTERFACE_H
 
 #include "os_compat.h"
+#include "gcc.h"
 
 struct client;
 
@@ -49,6 +50,16 @@ void client_write(struct client *client, const char *data, size_t length);
  */
 void client_puts(struct client *client, const char *s);
 
+/**
+ * Write a printf-like formatted string to the client.
+ */
+void client_vprintf(struct client *client, const char *fmt, va_list args);
+
+/**
+ * Write a printf-like formatted string to the client.
+ */
+mpd_fprintf void client_printf(struct client *client, const char *fmt, ...);
+
 int client_print(int fd, const char *buffer, size_t len);
 
 #endif