Patch: Fix libunwind segfault when -p is passed before -k

[Sean Stangl]

There is a segfault if strace is called as `strace -p 2260 -k`.

The -k flag sets a global bool stack_trace_enabled. But -p already consults
stack_trace_enabled during tcb initialization. So if -p is passed first,
the libunwind components of the tcb are uninitialized.

The attached patch fixes this issue by having -k ensure that the libunwind
components are initialized.

Best,
Sean Stangl
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