chasquid: De-couple TLS certificates from domains

Having the certificates inside the domain directory may cause some confusion,
as it's possible they're not for the same name (they should be for the MX we
serve as, not the domain itself).

So it's not a problem if we have domains with no certificates (we could be
their MX with another name), and we could have more than one certificate per
"domain" (if we act as MXs with different names).

So this patch moves the certificates out of the domains into a new certs/
directory, where we do a one-level deep lookup for the files.

While at it, change the names of the files to "fullchain.pem" and
"privkey.pem", which match the names generated by the letsencrypt client, to
make it easier to set up.  There's no general convention for these names
anyway.

test/t-02-exim/msmtprc

@@ -4,7 +4,7 @@ host srv-chasquid
 port 1587
 
 tls on
-tls_trust_file config/domains/srv-chasquid/cert.pem
+tls_trust_file config/certs/srv-chasquid/fullchain.pem
 
 from user@srv-chasquid