Further fallback logic refinement

Use macros to set and test early fallback in TLS policy lookup.

Move verification checks into new smtp_tls_trouble() which returns
non-zero when unrecoverable TLS problems prevent delivery over the
current connection.

Update smtp_tls_trouble() handling of the other TLS errors.  We
no longer update the tls policy object level or fallback level,
just perform all required logging and return the error indication.