Lookup tty to be used just once #1683

At the moment we're going through ttyname_from_dev in multuple places through current_tty_name, while this should be computed just once and used everywhere, avoiding issues such as the one fixed by #1682.

So I'd suggest to compute just once and keep it in the context to be used in the various places we need.

In fact right now we may end up computing the same value up to 5 times for a single execution:

• init_pam: for PAM_TTY
• log_command_execution: for the TTY= log entry
• RecordScope::for_tty: for the session record scope
• ttyname_from_dev: calls Process::tty_device_id() again on its own, so every current_tty_name() call reads /proc/self/stat again and may scan /dev/pts and /dev