tty: Default to printing kernel stack traceback only on INVARIANT kernels

Change the default from printing a breif kernel thread stack informaton
back to omitting it for non-invariant kernels in response to
SIGINFO/^T. Full and brief stack support can be selected with the
kern.tty_info_kstacks sysctl.

MFC After: 2 weeks
Sponsored by: Netflix
Reviewed by: grembo, jhb
Differential Revision: https://reviews.freebsd.org/D35576