More preparatory work: remove the <windows.h> include from lots of

source files in which it's no longer required (it was previously
required in anything that included <putty.h>, but not any more).
Also moved a couple of stray bits of exposed WinSock back into
winnet.c (getservbyname from ssh.c and AF_INET from proxy.c).

git-svn-id: svn://svn.tartarus.org/sgt/putty@2160 cda61777-01e9-0310-a592-d414129be87e