This change adds support for doing an SMTP STARTTLS dance before a TLS
handshake when using the tool. This is useful for poking at SMTP
servers.
Change-Id: I04cd60d02d3377cce83e412d62e3257235a19116
Reviewed-on: https://boringssl-review.googlesource.com/8662
Reviewed-by: David Benjamin <davidben@google.com>