mscp_prepare() scans source paths in a thread.

This commit runs mscp_prepare() in a pthread. mscp copy threads
run aysnchronously with mscp_prepare(). So, when mscp_prepare()
has not finished yet (due to too many source files), we can start
to copy files.

src/main.c

@@ -358,7 +358,7 @@ int main(int argc, char **argv)
 	}
 
 	if (dryrun) {
-		ret = 0;
+		ret = mscp_prepare_join(m);
 		goto out;
 	}