Rewrite local-proxy system to allow interactive prompts.

This fills in the remaining gap in the interactive prompt rework of
the proxy system in general. If you used the Telnet proxy with a
command containing %user or %pass, and hadn't filled in those
variables in the PuTTY config, then proxy/telnet.c would prompt you at
run time to enter the proxy auth details. But the local proxy command,
which uses the same format_telnet_command function, would not do that.
Now it does!

I've implemented this by moving the formatting of the proxy command
into a new module proxy/local.c, shared between both the Unix and
Windows local-proxy implementations. That module implements a
DeferredSocketOpener, which constructs the proxy command (prompting
first if necessary), and once it's constructed, hands it to a
per-platform function platform_setup_local_proxy().

So each platform-specific proxy function, instead of starting a
subprocess there and then and passing its details to make_fd_socket or
make_handle_socket, now returns a _deferred_ version of one of those
sockets, with the DeferredSocketOpener being the thing in
proxy/local.c. When that calls back to platform_setup_local_proxy(),
we actually start the subprocess and pass the resulting fds/handles to
the deferred socket to un-defer it.

A side effect of the rewrite is that when proxy commands are logged in
the Event Log, they now get the same amenities as in the Telnet proxy
type: the proxy password is sanitised out, and any difficult
characters are escaped.

proxy/proxy.h

@@ -105,6 +105,12 @@ prompts_t *proxy_new_prompts(ProxySocket *ps);
 char *format_telnet_command(SockAddr *addr, int port, Conf *conf,
                             unsigned *flags_out);
 
+DeferredSocketOpener *local_proxy_opener(
+    SockAddr *addr, int port, Plug *plug, Conf *conf, Interactor *itr);
+void local_proxy_opener_set_socket(DeferredSocketOpener *opener,
+                                   Socket *socket);
+char *platform_setup_local_proxy(Socket *socket, const char *cmd);
+
 #include "cproxy.h"
 
 #endif