tty info (^T): Add optional kernel stack(9) traces

It is often useful for developers and administrators to determine a running
thread's stack for debugging purposes. With this feature, using ^T will
print that information

For now, the feature is disabled by default. Enable with sysctl
kern.tty_info_kstacks=1.

Discussed with: markj
Reviewed by: oshogbo
Relnotes: yes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17621