Acquire/release a POSIX file lock on repository archives.

- xbps_repo_open() accepts a third argument (bool) to acquire a POSIX file
lock on the repository archive.
- xbps_repo_close() accepts a second argument (bool) to release a POSIX file
lock on the repository archive.

This avoids the issue of multiple xbps-rindex(8) processes being blocked
even for different repositories on the same architecture, resulting in
unnecessary contention.

include/xbps.h.in

@@ -48,7 +48,7 @@
  *
  * This header documents the full API for the XBPS Library.
  */
-#define XBPS_API_VERSION	"20140812"
+#define XBPS_API_VERSION	"20140905"
 
 #ifndef XBPS_VERSION
  #define XBPS_VERSION		"UNSET"
@@ -1245,6 +1245,10 @@ struct xbps_repo {
 	 * URI string associated with repository.
 	 */
 	const char *uri;
+	/**
+	 * @private
+	 */
+	int fd;
 	/**
 	 * var is_remote
 	 *
@@ -1372,17 +1376,19 @@ xbps_dictionary_t xbps_rpool_get_pkg_plist(struct xbps_handle *xhp,
  *
  * @param[in] xhp Pointer to the xbps_handle struct.
  * @param[in] uri Repository URI to match.
+ * @param[in] lock Set it to true to acquire a POSIX file lock.
  *
  * @return The matching repository object, NULL otherwise.
  */
-struct xbps_repo *xbps_repo_open(struct xbps_handle *xhp, const char *url);
+struct xbps_repo *xbps_repo_open(struct xbps_handle *xhp, const char *url, bool lock);
 
 /**
  * Closes a repository object and releases resources.
  *
  * @param[in] repo The repository object to close.
+ * @param[in] lock Set it to true to release the POSIX file lock.
  */
-void xbps_repo_close(struct xbps_repo *repo);
+void xbps_repo_close(struct xbps_repo *repo, bool lock);
 
 
 /**