new index and search commands

now it is possible to index all filenames contained in tombs using
updatedb(8) and then automatically search for them using locate in all
open tombs. Documentation was updated accordingly.
Security check for extra keyslots

when a tomb is opened we check if other slots besides the master (0)
are enabled and if so we warn the user that there can be a backdoor.
LUKS permits the presence of multiple keys that can open a single
volume, but this weakens the security of it and we are not really
interested in implementing it.
create command restored for backward compatibility

the new mechanism to create tombs is in place (dig/forge/lock), but we
still wrap it with a create command for backward compatibility.
small fix in password processing