Be more consistent about sockets and blocking/non-blocking mode:

Be more consistent about sockets and blocking/non-blocking mode:

Accept sockets are non-blocking, to avoid races where the client closes
before we get to accept it.  (Spotted by: "chen xiaoyong")

Unfortunately, that means that session sockets inherit non-blocking mode,
which is the opposite of what we want in the worker thread but correct
for the acceptor thread.

We prefer to have the extra syscalls in the worker thread, that complicates
things a little bit.

Use ioctl(FIONBIO) instead of fcntl(2) which is surprisingly expensive.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2639 d4fa192b-c00b-0410-8231-f00ffab90ce4