imap: Added support for the STARTTLS capability (Part One)

Introduced detection of the STARTTLS capability, in order to add support
for TLS upgrades without unconditionally sending the STARTTLS command.
pop3: Added support for the STLS capability (Part One)

Introduced detection of the STLS capability, in order to add support
for TLS upgrades without unconditionally sending the STLS command.
smtp: Added support for the STLS capability (Part One)

Introduced detection of the STARTTLS capability, in order to add support
for TLS upgrades without unconditionally sending the STARTTLS command.
imap: Added support for the STARTTLS capability (Part Two)

Added sending of initial CAPABILITY command before STARTTLS is sent.
This allows for the detection of the capability before trying to
upgrade the connection.
pop3: Added support for the STLS capability (Part Two)

Added sending of initial CAPA command before STLS is sent. This allows
for the detection of the capability before trying to upgrade the
connection.