ssh ControlMaster: get the exact %C socket path from ssh -G; retries leak logins

Tested with OpenSSH 9.6p1. ssh -G <same args> host | awk '/^controlpath /{print $2}' prints the fully expanded ControlPath (the %C hash resolved), and the path changes when the args change resolved values (e.g. -p). Use it as the key for connection pools and for -O check/-O exit, instead of reimplementing the hash or keying on the unexpanded template. %C is 'Hash of %l%h%p%r%j', so leaving out an option that equals the default (e.g. -l with your own user) still finds the master, but a missing -p doesn't (rc=255, 'Control socket connect(...): No such file'). Restarting with -o ControlMaster=yes -fN while a master holds the socket prints 'ControlSocket ... already exists, disabling multiplexing' and exits 0, but the backgrounded client stays connected as an extra login. Every such retry leaks one session. Add -o ClearAllForwardings=yes to background masters, or LocalForward entries from ssh_config get opened.