Prefer signed types in blocking_read etc

Prefer signed types in blocking_read etc

* src/compare.c (process_noop, process_rawdata):
Return bool, not int.
* src/compare.c (process_noop, process_rawdata):
* src/create.c (dump_regular_file):
* src/extract.c (extract_file):
* src/misc.c (blocking_read, blocking_write):
* src/sparse.c (sparse_scan_file_raw, sparse_extract_region):
Prefer signed types like idx_t to unsigned ones like size_t.
(sparse_scan_file_raw): Diagnose read errors.