cp: refactor out data copying

cp: refactor out data copying

* po/POTFILES.in, src/local.mk (copy_sources): Add the new file.
* src/copy.c: Move the #includes of alignalloc.h, buffer-lcm.h,
fadvise.h, full-write.h, ioblksize.h to copy-file-data.c.
(enum copy_debug_val, struct copy_debug): Move these decls to copy.h.
(punch_hole, create_hole, is_CLONENOTSUP, sparse_copy)
(write_zeros, lseek_copy, HAVE_STRUCT_STAT_ST_BLOCKS)
(enum scantype, struct scan_inference, infer_scantype):
Move to copy-file-data.c.
(copy_reg): Move the data-copying part of this function
to the new function copy_file_data in copy-file-data.c.
* src/copy-file-data.c: New file, taken from part of copy.c.