Currently the "add" action adds the token to all keyslots, this
changes the behaviour to make sure the token is added to the first
keyslot that can be unlocked using the provided passphrase.
Test needs to work also in build environment
before installation takes place. For it we overload
crypt_token_external_path symbol from libcryptsetup
so that it can look for plugins in build environment.
[simplified by mbroz]