linuxkpi: Move string_helpers.h to string_choices.h

Chace Linux change fca76071bab2 ("lib/string_helpers: Split out
string_choices.h") from v6.5. Everything that we implemented from
string_helpers.h has moved, so rename the file, update the include
guards, and add a new dummy string_helpers.h that just #includes it.
This matches Linux for the functionality that we have.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52606