LinuxKPI: Add str_on_off and other helpers to linux/string_helpers.h

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz, imp (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42794

(cherry picked from commit ae1084ff01bf9d9a9a2c56caf6c7095f3e96f579)