priority: support allowlisting in configuration file

priority: support allowlisting in configuration file

This adds a new mode of interpreting the [overrides] section.  If
"override-mode" is set to "allowlisting" in the [global] section, all
the algorithms (hashes, signature algorithms, curves, and versions)
are initially marked as insecure/disabled.  Then the user can enable
them by specifying allowlisting keywords such as "secure-hash" in the
[overrides] section.

Signed-off-by: Daiki Ueno <ueno@gnu.org>
Co-authored-by: Alexander Sosedkin <asosedkin@redhat.com>