Make xbps_fetch_file accept a callback to update its progress.

This also makes xbps_unpack_binary_pkg follow this convention by avoiding
static variables.

--HG--
branch : progress_callback
rename : bin/xbps-repo/util.c => bin/xbps-bin/util.c

bin/xbps-repo/main.c

@@ -34,6 +34,7 @@
 
 #include <xbps_api.h>
 #include "defs.h"
+#include "../xbps-bin/defs.h"
 
 static void __attribute__((noreturn))
 usage(void)