Add a new advanced command mode to s_client

The s_client command mode just looks at the first letter on a line
and if it is one that maps to a "command" then that command is executed
and everything else on the line is ignored.

This means it is impossible to send anything over s_client that starts with
one of the "magic" letters unless you switch off command processing
entirely.

With the forthcoming quic support that will be added to s_client we expect
a significant new number of commands being added (for example to send fin
on a stream, or to list or swap between different streams, etc). This will
use up more "letters" and reduces the usability of s_client.

This PR adds a new "advanced" command processing mode. Commands are enclosed
in braces {}, and can appear anywhere in the text typed into s_client. A
brace can be escaped by sending a double {{. This gives much more
flexibility without reducing the usability of s_client. Commands can also
have "arguments" which also extends the scope of what they can be used for.

Reviewed-by: Tomas Mraz <[email protected]>
Reviewed-by: Paul Dale <[email protected]>
(Merged from openssl#20566)