Revise archive procedure in gatekeeper

Revise archive procedure in gatekeeper

 - a structured exception for general processing errors is added
 - the archive sub is renamed to archive_filepair
 - a file and its detached signature are now archived together
 - the archived filename now contains an extra number beyond the timestamp
   only if actually needed for uniqueness
   - the extra number, if used, matches between a file and its signature
   - the archived filename is claimed by creating an "archive stamp" file
 - the option to archive and overwrite a file using "replace" now handles
   the file and its signature as a pair
 - the system mkdir(1) and mv(1) commands are no longer invoked here
 - the testsuite is adjusted accordingly