crash_prog was introduced to force-crash a userspace program
since _exit may not yet be implemented. However, the new versions
of OS161 already have the exact same logic as crash_prog in stdlib/exit.
and userspace.
In userspace, this is accessed through #include <secure.h>
In the kernel, this is access through #include <kern/secure.h>
There is a unit test for this (hm1) that computes the hmac and compares it to
the known value. The salted vesion tested offline.
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Also, fixed usespace compile issue with not changing KERNEL_SECRET => SECRET.
