Move all extern declarations into header files.

This is another cleanup I felt a need for while I was doing
boolification. If you define a function or variable in one .c file and
declare it extern in another, then nothing will check you haven't got
the types of the two declarations mismatched - so when you're
_changing_ the type, it's a pain to make sure you've caught all the
copies of it.

It's better to put all those extern declarations in header files, so
that the declaration in the header is also in scope for the
definition. Then the compiler will complain if they don't match, which
is what I want.

windows/winproxy.c

@@ -12,9 +12,6 @@
 #include "network.h"
 #include "proxy.h"
 
-Socket *make_handle_socket(HANDLE send_H, HANDLE recv_H, HANDLE stderr_H,
-                           Plug *plug, bool overlapped);
-
 Socket *platform_new_connection(SockAddr *addr, const char *hostname,
                                 int port, bool privport,
                                 bool oobinline, bool nodelay, bool keepalive,