pthread_getattr_np was reporting the values supplied to us, not the values we
actually used, which is kinda the whole point of pthread_getattr_np.
pthread_attr_setguardsize and pthread_attr_setstacksize were reporting EINVAL
for any size that wasn't a multiple of the system page size. This is
unnecessary. We can just round like POSIX suggests and glibc already does.
Also improve the error reporting for pthread_create failures.
Change-Id: I7ebc518628a8a1161ec72e111def911d500bba71
b95cf0d23a