Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>

Try to make the description and the instructions clearer.

In the README file, provide the detailed steps for typical use cases.
Mention that the alphabetical order is case-sensitive, and that the
checksum files are compatible with sha256sum command.

In the help message, instead of describing the details of each command,
first describe what the expected status is, because it will be checked
by all commands. It simplifies the description of the commands.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>

Read the last modification time of each file. If a file is already
listed, but more recent than the checksum file, then it means that the
checksum file is not up to date for this file.

In that case, the file must not be considered altered (bad checksum).
It has obviously been deliberately modified after the checksum file was
created or updated for the last time.

* When using the 'list' or 'check' command, a warning must be printed
  (with the 'check' command it is not even necessary to verify the
  checksum).
* When using the 'update' command, the checksum of the file must be
  updated.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>

When the checksum file is created or updated, make sure to set the
read-only permission on it. This file should not be manually edited.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>

With this first version you can create checksum files, update them if
some files are added or removed, and verify the checksums.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>