usr.bin/sort: Avoid UBSan errors

UBSan complains about out-of-bounds accesses for zero-length arrays. To
avoid this we can use flexible array members. However, the C standard does
not allow for structures that only contain flexible array members, so we
move the length parameters into that structure too.

Split out from D28233.

Reviewed By: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31009

(cherry picked from commit d053fb22f6d3b6bb0f4e47e4507fefc20cbe0e32)