sysctl(9): fix handling string tunables.

r357614 changed internals of handling string sysctls, and inadvertently
broke setting string tunables. Take them into account.

PR: 245463
Reported by: jhb, np
Reviewed by: imp, jhb, kib
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D24429