fs-util: add calls that combine chase_symlinks() and open()/opendir() in one

This is useful when opening files within disk images, as we'll then take
the relative root directory properly into account.

src/basic/fs-util.h

@@ -20,6 +20,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <dirent.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <stdbool.h>
@@ -91,6 +92,9 @@ enum {
 
 int chase_symlinks(const char *path_with_prefix, const char *root, unsigned flags, char **ret);
 
+int chase_symlinks_and_open(const char *path, const char *root, unsigned chase_flags, int open_flags, char **ret_path);
+int chase_symlinks_and_opendir(const char *path, const char *root, unsigned chase_flags, char **ret_path, DIR **ret_dir);
+
 /* Useful for usage with _cleanup_(), removes a directory and frees the pointer */
 static inline void rmdir_and_free(char *p) {
         PROTECT_ERRNO;