Make pid files backwards compatible and add address of listening socket.

The pid is now written first, so that a version 1.0.x tincd can be used to stop
a running version 1.1 tincd.  Getsockname() is used to determine the address of
the first listening socket, so that tincctl can connect to the local tincd even
if AddressFamily = ipv6, or if BindToAddress or BindToInterface is used.