There are special rules regarding the memory passed to pthread_attr_setstack().
ok semarie@

Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_att

Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.
This opportunistic software-emulation of a stack protection bit makes
stack-pivot operations during ROPchain fragile (kind of like removing a
tool from the toolbox).
original discussion with tedu, uvm work by stefan, testing by mortimer
ok kettenis

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use fancy .In macro for includes. from Jan Klemkow. ok jmc schwarze

Add pthread_attr_[sg]etguardsize() to match rthread, including manpages
Bump lib minor

ok otto@ kurt@ marc@; doc review by jmc@

- correct .Dt for pthread_attr_setstack.3
- new sentence, new line

Add manpage for pthread_attr_[sg]etstack(3)
Correct misnaming in STANDARDS section
ok brad@ otto@