copy: teach copy_bytes() sendfile() support, and then replace sendfile_full() by it

src/shared/copy.h

@@ -24,6 +24,7 @@
 #include <stdbool.h>
 #include <sys/types.h>
 
+int copy_file_fd(const char *from, int to);
 int copy_file(const char *from, const char *to, int flags, mode_t mode);
 int copy_tree(const char *from, const char *to, bool merge);
 int copy_bytes(int fdf, int fdt, off_t max_bytes);