Revise and rename keyring_file to directory_keyrings

Revise and rename keyring_file to directory_keyrings

This fixes a long-standing bug, due to keyring_file never actually
returning an empty list, even if the "root" keyring does not actually
exist in the filesystem.  The testsuite is adjusted accordingly.

This also introduces another minor issue, in that processing for a
misconfigured package is now abandoned earlier, before email addresses
are gathered.  This will be corrected in later improvements.

This also eliminates the special handling for a "root" keyring, although
the feature remains available as part of the general case:  "pubring.gpg"
at the root of the package configuration tree applies to all directories
in all packages.  Be very careful with the keys on such a ring, as they
would be of immense value to an attacker.