Move sockaddr_is_loopback() to before sk_address_is_local(), and define the

Move sockaddr_is_loopback() to before sk_address_is_local(), and define the
latter in terms of the former.  Also adjust the definition of
ipv4_is_loopback() to avoid using the non-standard inet_netof() and
IN_LOOPBACKNET, and move it next to its remaining uses.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5215 cda61777-01e9-0310-a592-d414129be87e