Instead of incorrectly assuming that a pthread_t can be meaningfully cast

Instead of incorrectly assuming that a pthread_t can be meaningfully cast
to an unsigned int (which triggered warnings on some 64-bit platforms) and
printed with %08lx, incorrectly assume that it can be meaningfully cast to
a void * and printed with %p.  While still incorrect in general terms, the
latter turns out to be correct on the specific systems that we care about.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1754 d4fa192b-c00b-0410-8231-f00ffab90ce4