terminal-util: stop doing 0/upper bound check in tty_is_vc()

terminal-util: stop doing 0/upper bound check in tty_is_vc()

tty_is_vc() is more often than not used for simple "categorization"
than validity check. E.g. in logind, we first recognize the tty
"looks like vc", and then use vtnr_from_tty() where range check
is performed and vtnr is extracted. In such cases, we want to reject
invalid vtnr from clients rather than silently carry on, hence
let's remove bound check in tty_is_vc().

Fixes #36166
Replaces #36167 and #36175