Support for doing DNS at the proxy end. I've invented a new type of

Support for doing DNS at the proxy end. I've invented a new type of
SockAddr, which just contains an unresolved hostname and is created
by a stub function in *net.c. It's an error to pass this to most of
the real-meat functions in *net.c; these fake addresses should have
been dealt with by the time they get down that far. proxy.c now
contains name_lookup(), a wrapper on sk_namelookup() which decides
whether or not to do real DNS, and the individual proxy
implementations each deal sensibly with being handed an unresolved
address and avoid ever passing one down to *net.c.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2353 cda61777-01e9-0310-a592-d414129be87e