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/firstboot/firstboot.c

@@ -384,7 +384,7 @@ static int process_locale(int rfd, sd_varlink **mute_console_link) {
                 return log_error_errno(r, "Failed to check if directory file descriptor is root: %m");
 
         if (arg_copy_locale && r == 0) {
-                r = copy_file_atomic_at(AT_FDCWD, etc_locale_conf(), pfd, f, 0644, COPY_REFLINK);
+                r = copy_file_atomic_at(AT_FDCWD, etc_locale_conf(), pfd, f, 0644, /* copy_flags= */ 0);
                 if (r != -ENOENT) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to copy host's /etc/locale.conf: %m");
@@ -526,7 +526,7 @@ static int process_keymap(int rfd, sd_varlink **mute_console_link) {
                 return log_error_errno(r, "Failed to check if directory file descriptor is root: %m");
 
         if (arg_copy_keymap && r == 0) {
-                r = copy_file_atomic_at(AT_FDCWD, etc_vconsole_conf(), pfd, f, 0644, COPY_REFLINK);
+                r = copy_file_atomic_at(AT_FDCWD, etc_vconsole_conf(), pfd, f, 0644, /* copy_flags= */ 0);
                 if (r != -ENOENT) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to copy host's /etc/vconsole.conf: %m");