POSIX compatible exit codes, minor cosmetics, new checks added to "make check"

File hashing, rather than checking hashes etc, now returns POSIX compatible exit codes (0 means 'success').

Some minor changes, a number of functions now accept 'const char' rather than 'char' strings.

Checks added to 'make check' to test that hashing returns correct exit codes, and to test hashing from a list of files, either a file containing the list, or a list fed into stdin.