Add a flag to ignore file modification time to archive-tool check

[RKrahl]

The archive-tool check subcommand checks whether some files are in a archive. A file is considered to be in the archive if the file size and checksum matches and the file modification time is not newer than modification time recorded in the archive for that entry.

Now, the modification time for a file is somewhat fragile, it may for instance be set by merely copying the file. It might be useful in some cases to be able to ignore that criterion in the checks. A command line flag like --ignore-mtime should be added to the archive-tool check subcommand for this purpose.