Allow vcprintf() to be called by external code such as the curses library.

Also trim another eight bytes from vsprintf.o.  :)

src/include/vsprintf.h

@@ -35,6 +35,35 @@
 
 #define PRINTF_NO_LENGTH ( ( size_t ) -1 )
 
+/**
+ * A printf context
+ *
+ * Contexts are used in order to be able to share code between
+ * vprintf() and vsnprintf(), without requiring the allocation of a
+ * buffer for vprintf().
+ */
+struct printf_context {
+	/**
+	 * Character handler
+	 *
+	 * @v ctx	Context
+	 * @v c		Character
+	 *
+	 * This method is called for each character written to the
+	 * formatted string.
+	 */
+	void ( * handler ) ( struct printf_context *ctx, unsigned int c );
+	/** Length of formatted string
+	 *
+	 * When handler() is called, @len will be set to the number of
+	 * characters written so far (i.e. zero for the first call to
+	 * handler()).
+	 */
+	size_t len;
+};
+
+extern size_t vcprintf ( struct printf_context *ctx, const char *fmt,
+			 va_list args );
 extern int vsnprintf ( char *buf, size_t size, const char *fmt, va_list args );
 extern int vprintf ( const char *fmt, va_list args );