Simplify the interface into httpserver and UnixHttpServer -- the root and

Simplify the interface into httpserver and UnixHttpServer -- the root and
interface parameters are always used, so there's no need for them to be named
parameters with defaults.

Remove unused httpserver.getRoot and getPort, and simplify the main request
loop.  This means that socket errors do not have to be squelched.

Coalesce the two identical bind methods, one in UnixHttpServer, one in
UnixListener.  Fix this bind method to set the permissions on the socket
explicitly.  This closes a security hole, and fixes the intermittent failure
of xm-test/06_list_nonroot.test.

Signed-off-by: Ewan Mellor <ewan@xensource.com>