sed: treat '[' as ordinary character in 'y' command

'y' does not handle bracket expressions, treat '[' as ordinary character
and do not apply bracket expression checks (GNU sed agrees).

PR: 247931
Reviewed by: pfg, kevans
Tested by: antoine (exp-run), Quentin L'Hours <lhoursquentin@gmail.com>
Differential Revision: https://reviews.freebsd.org/D25640