Add Client* ClientManager::get_client_ifp() that can return nullptr
eval and exec can use a -try-client option

When -try-client <client_name> is used, the context used for the
command is either the given client's context if the client is found,
or the  current one if not.
use -try-client in kak files rather than ad-hoc shell logic
ctags.kak: Show message with Information colalias in gentags