Improvements for xbps_fetch_file and xbps_repository_sync_index.

xbps_fetch_file:
	- A temp file is created with .part extension to improve resuming.
	- Files are downloaded in cwd.
	- Switch to futimens(2) and fsync(2).

xbps_repository_sync_index:
	- Do not create local repodir in metadir if it already exists.
	- Simplify the code thanks to new xbps_fetch_file().

lib/external/fexec.c

@@ -27,6 +27,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define _BSD_SOURCE	/* for vfork and chroot */
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <errno.h>