AIX has a "struct thread" in pthread.h, which conflicts with our
struct thread.  Rename ours to replay_thread.

Fixes #531

Thanks to demik for patch

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4152 d4fa192b-c00b-0410-8231-f00ffab90ce4