abstractions/ssh_certs: extend pki/trust directories
- add pki/blacklist and pki/blocklist
- add /usr/share/pki/ in adddition to /etc/pki/
pki/blocklist was suggested by @darix, the other changes are things I noticed while adding it.
I propose this patch for 3.0 and master. (abstractions/ssl_certs
on 2.x branches is quite different and needs a manual backport (or more cherry-picks) if you want to backport this MR.)