TELNET option IAC injection

curl supports communicating using the TELNET protocol and as a part of this it
offers users to pass on username and “telnet options” for the server
negotiation.

Due to lack of proper input scrubbing and without it being the documented
functionality, curl would pass on username and telnet options to the server
as provided. This could allow users to pass in carefully crafted content that
pass on content or do option negotiation without the application intending to
do so. In particular if an application for example allows users to provide the
data or parts of the data.