* thread.cc (pthread::create): Use PTHREAD_DEFAULT_STACKSIZE stacksize
if attr.stacksize is 0.
(pthread_attr::pthread_attr): Initialize stacksize to 0 to align more
closely to Linux.
(pthread_attr_getstack): Fix incorrect stackaddr computation. Return
stackaddr just like pthread_attr_getstackaddr. Remove slightly off
comment.
(pthread_attr_getstackaddr): Remove slightly off comment.
(pthread_getattr_np): Return stackaddr and stacksize based on the full
allocated stackarea.