Use implicit TLS as default for every protocol.

Implicit TLS is more secure and more performant than STARTTLS. Thus, Balsa should use implicit TLS as the default for SMTP (port 465), POP3 (port 995), and IMAP (port 993).

Currently, it uses STARTTLS for POP3 and IMAP. On my NixOS machine, SMTP is not automatically configured, which could also be an unrelated (?) bug. Still, I think SMTP should also default to implicit TLS on port 465.