- add --is_gzip option to sample_bitwise_validator.
If set, all files are treated as gzip archives.
Check their 10-byte header to verify that it's a gzip file,
but ignore it when comparing files.
- validator.cpp: don't error out on unparsed cmdline args,
since we're now using them in sample_bitwise_validator
and sample_substr_validator.
- fix build error on Debian
David Anderson