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/sysusers/sysusers.c

@@ -333,7 +333,7 @@ static int make_backup(const char *target, const char *x) {
         if (r < 0)
                 return r;
 
-        r = copy_bytes(src, fileno(dst), UINT64_MAX, COPY_REFLINK);
+        r = copy_bytes(src, fileno(dst), UINT64_MAX, /* copy_flags= */ 0);
         if (r < 0)
                 return r;