xbps-install(1): added -D, --download-only.

Added `-D, --download-only` flag to allow downloading
packages to the cache without attempting to install them.

Contributed by Toyam Cox via #9

Adapted to master by yours truly.

Close #9

include/xbps.h.in

@@ -210,11 +210,18 @@
 #define XBPS_FLAG_UNPACK_ONLY 		0x00001000
 
 /**
+ * @def XBPS_FLAG_DOWNLOAD_ONLY
+ * Only download packages to the cache, do not do any installation steps.
+ * Must be set through the xbps_handle::flags member.
+ */
+#define XBPS_FLAG_DOWNLOAD_ONLY		0x00002000
+
+/*
  * @def XBPS_FLAG_IGNORE_FILE_CONFLICTS
  * Continue with transaction even if there are file conflicts.
  * Must be set through the xbps_handle::flags member.
  */
-#define XBPS_FLAG_IGNORE_FILE_CONFLICTS	0x00002000
+#define XBPS_FLAG_IGNORE_FILE_CONFLICTS	0x00004000
 
 /**
  * @def XBPS_FETCH_CACHECONN
@@ -1282,7 +1289,6 @@ xbps_dictionary_t xbps_archive_fetch_plist(const char *url, const char *p);
 /** @addtogroup repopool */
 /*@{*/
 
-
 /**
  * @struct xbps_repo xbps.h "xbps.h"
  * @brief Repository structure
@@ -1633,7 +1639,6 @@ xbps_array_t xbps_repo_get_pkg_revdeps(struct xbps_repo *repo, const char *pkg);
  */
 int xbps_repo_key_import(struct xbps_repo *repo);
 
-
 /*@}*/
 
 /** @addtogroup archive_util */