copy: optionally, reproduce hardlinks from source in destination

This is useful for duplicating trees that contain hardlinks: we keep
track of potential hardlinks and try to reproduce them within the
destination tree. (We do not hardlink between source and destination!).

This is useful for trees like ostree images which heavily use hardlinks
and which are otherwise exploded into separate copies of all files when
we duplicate the trees.

src/basic/copy.h

@@ -17,6 +17,7 @@ typedef enum CopyFlags {
         COPY_CRTIME      = 1 << 5, /* Generate a user.crtime_usec xattr off the source crtime if there is one, on copying */
         COPY_SIGINT      = 1 << 6, /* Check for SIGINT regularly and return EINTR if seen (caller needs to block SIGINT) */
         COPY_MAC_CREATE  = 1 << 7, /* Create files with the correct MAC label (currently SELinux only) */
+        COPY_HARDLINKS   = 1 << 8, /* Try to reproduce hard links */
 } CopyFlags;
 
 typedef int (*copy_progress_bytes_t)(uint64_t n_bytes, void *userdata);