- use PATH_SEPARATOR everywhere,
instead of a mix of PATH_SEPARATOR and hard coded colons.
- always include /usr/share/yubikey-val first,
then /etc/yubico/val.
- should not have any visible affects,
since no file names are common between the two directories.
- use array+implode to make it easier to add/remove paths,
and to avoid hardcoded the path separator.
