Nonce::{cc_str,data}: Make const

Found by cppcheck.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Dispatcher::get_{dispatch_chars,OSC_string}: Make const

Found by cppcheck

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
STMClient::still_connecting: Make const

Found by cppcheck.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>