More tls_sni support: outbound, logging.

tls_sni as SMTP transport option.
Use correct storage pool for copying tls_sni, so survives for life of process.
Add +tls_sni log-selector, for inbound tls_sni.
Update exipick to handle -tls_sni in spool files.

Also reset tls_bits at start of outbound connection (was missing).