xz: Improve compatibility with systems without capability mode support

When the kernel is built without capability mode support, or when
using an emulator like qemu-user-static that does not translate
system calls, these calls will return a negative number and set
the errno to ENOSYS. However, this error does not indicate a
real programming or runtime error and is generally ignored by
base system applications built with capability mode sandboxing.

Match this behavior by making xz(1) to ignore ENOSYS errors
when calling capability mode system calls too.

PR: 269185
Reported by: Dan Kotowski
MFC after: 2 days