Fix the t_join test

For pthread_attr_get_np() attr should be initialized prior to the call by
using pthread_attr_init(3). pthread_getattr_np() does this
automatically so switch to it.

Export the guard size of the main thread via vm.guard_size. Add a
complementary writable sysctl for the initial guard size of threads
created via pthread_create. Let the existing attribut accessors d

Export the guard size of the main thread via vm.guard_size. Add a
complementary writable sysctl for the initial guard size of threads
created via pthread_create. Let the existing attribut accessors do the
right thing. Raise the default guard size for threads to 64KB.

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Also exercise pthread_attr_setstacksize.

Remove bogus check that (a) depends uninitialised memory (b) overflows a
buffer.

Requires stdint.h.

Test alignment of constructor / destructor calls as well as the stack
of new threads. Currently implement for i386 and AMD64.

Revert to 1.1 and fix the casts by using uintptr_t. Ok jruoho.

One more cast.

Fix build failure on AMD64, noted by njoly@.

Add a simple test for pthread_join(3).