rcon: rewrite variable expansion support, related fixes

rcon: rewrite variable expansion support, related fixes

Fixes https://gitlab.com/xonotic/darkplaces/-/issues/407
In 48d83538ef13596d4569d46ae66467b63b945546 I broke the return of the
rcon command output to the client.

Fixes unterminated string warnings on the server when the
rcon_password feature of multiple space-delimited passwords is used.
Fixes possibility of the cbuf to use more memory than its intended
maximum.

In the client you can send the variable such that the server will
expand it (not the client) like this: rcon echo $$sv_worldmessage

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>