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/tmpfiles/tmpfiles.c

@@ -2236,7 +2236,7 @@ static int copy_files(Context *c, Item *i) {
                          dfd, bn,
                          i->uid_set ? i->uid : UID_INVALID,
                          i->gid_set ? i->gid : GID_INVALID,
-                         COPY_REFLINK | ((i->append_or_force) ? COPY_MERGE : COPY_MERGE_EMPTY) | COPY_MAC_CREATE | COPY_HARDLINKS,
+                         ((i->append_or_force) ? COPY_MERGE : COPY_MERGE_EMPTY) | COPY_MAC_CREATE | COPY_HARDLINKS,
                          NULL, NULL);
 
         fd = openat(dfd, bn, O_NOFOLLOW|O_CLOEXEC|O_PATH);