cp: prefer signed types in copy-file-data.c

cp: prefer signed types in copy-file-data.c

* src/copy-file-data.c (sparse_copy, lseek_copy)
(copy_file_data): Prefer idx_t to size_t.
(copy_file_data): Defend against st_size < 0, which can happen on
ancient buggy platforms.  Check for overflow; the old code was
wrong on theoretical-but-valid hosts where SIZE_MAX <= INT_MAX.