Make sigaltstack as per-threaded, because per-process sigaltstack state is useless for threaded programs, multiple threads can not share same stack. The alternative signal stack is private for thread, no lock is needed, the orignal P_ALTSTACK is now moved into td_pflags and renamed to TDP_ALTSTACK. For single thread or Linux clone() based threaded program, there is no semantic changed, because those programs only have one kernel thread in every process.
Make sigaltstack as per-threaded, because per-process sigaltstack state
is useless for threaded programs, multiple threads can not share same
stack.
The alternative signal stack is private for thread, no lock is needed,
the orignal P_ALTSTACK is now moved into td_pflags and renamed to
TDP_ALTSTACK.
For single thread or Linux clone() based threaded program, there is no
semantic changed, because those programs only have one kernel thread
in every process.

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