Support “-starttls smtp” in `bssl client`

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>
diff --git a/tool/transport_common.h b/tool/transport_common.h
index b701015..a57a916 100644
--- a/tool/transport_common.h
+++ b/tool/transport_common.h
@@ -39,5 +39,8 @@
 
 bool TransferData(SSL *ssl, int sock);
 
+// DoSMTPStartTLS performs the SMTP STARTTLS mini-protocol over |sock|. It
+// returns true on success and false otherwise.
+bool DoSMTPStartTLS(int sock);
 
 #endif  /* !OPENSSL_HEADER_TOOL_TRANSPORT_COMMON_H */