copy: drop COPY_REFLINK

Reflinking is a safe optimization whenever the source and destination are
regular, seekable files on a filesystem that supports cloning. Requiring each
caller to opt in through COPY_REFLINK adds flag plumbing without changing the
necessary fallback behavior.

Drop COPY_REFLINK, renumber the remaining flags, and have copy_bytes_full()
unconditionally try FICLONE or FICLONERANGE before entering its normal copy
loop. Isolate the clone attempt and its file-offset bookkeeping in a helper so
copy_bytes_full() only has to handle its cloned, unavailable, and error results.
A successful clone therefore completes in one operation, while an unsupported
or rejected clone falls back to progress-limited copy_file_range(), sendfile(),
or buffered copying. Keep the public reflink helpers for operations that
specifically require clone semantics.

Existing test-copy coverage exercises both bounded and unbounded regular-file
copies through copy_bytes_full().

Signed-off-by: Daan De Meyer <daan@amutable.com>

src/core/exec-invoke.c

@@ -3594,7 +3594,7 @@ static int setup_ephemeral(
                 log_debug("Making ephemeral copy of %s to %s", rootfs->image, new_root);
 
                 fd = copy_file(rootfs->image, new_root, O_EXCL, 0600,
-                               COPY_LOCK_BSD|COPY_REFLINK|COPY_CRTIME|COPY_NOCOW_AFTER);
+                               COPY_LOCK_BSD|COPY_CRTIME|COPY_NOCOW_AFTER);
                 if (fd < 0) {
                         *reterr_path = strdup(rootfs->image);
                         return log_debug_errno(fd, "Failed to copy image %s to %s: %m",