crc32sum: Add function digest_filestream(int fd)

digest_filestream() computes the CRC32 hash for the data pointed to
by the file descriptor fd.
crc32sum: Rename sum_file() to digest_file()
crc32sum: Fix macro STREQ
Makefile: Enable all warnings

Add gcc's -Wall flag to compilation recipe.
crc32sum: Fix warnings and some housekeeping
crc32sum: New function sum_file(), read stdin and input options

Rewrite main() to accept reading data from stdin and input options
--help and --version. Now, function sum_file() manages access to
the data digest functions.