Enable ssh agent authentication on Windows

Windows has supported unix domain sockets for a couple of years
now; see this article for more information about that:
<https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/>

This commit allows libssh to consider using agent authentication
on Windows systems.

It is mostly removing `#ifndef _WIN32` that prevented the unix
domain socket code from being compiled in, and adjusting the use
of `read(2)` and `write(2)` to `recv(2)` and `send(2)`, as the former
functions are not compatible with sockets on Windows systems.

For mingw systems, afunix.h isn't available so we use the
technique as was used to resolve building with mingw as used
by the curl project in: https://github.com/curl/curl/pull/5170

Signed-off-by: Wez Furlong <wez@fb.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/agent.h

@@ -77,7 +77,6 @@ struct ssh_agent_struct {
   ssh_channel channel;
 };
 
-#ifndef _WIN32
 /* agent.c */
 /**
  * @brief Create a new ssh agent structure.
@@ -115,6 +114,5 @@ ssh_key ssh_agent_get_first_ident(struct ssh_session_struct *session,
 ssh_string ssh_agent_sign_data(ssh_session session,
                                const ssh_key pubkey,
                                struct ssh_buffer_struct *data);
-#endif
 
 #endif /* __AGENT_H */

include/libssh/socket.h

@@ -35,9 +35,9 @@ void ssh_socket_reset(ssh_socket s);
 void ssh_socket_free(ssh_socket s);
 void ssh_socket_set_fd(ssh_socket s, socket_t fd);
 socket_t ssh_socket_get_fd(ssh_socket s);
-#ifndef _WIN32
 int ssh_socket_unix(ssh_socket s, const char *path);
 void ssh_execute_command(const char *command, socket_t in, socket_t out);
+#ifndef _WIN32
 int ssh_socket_connect_proxycommand(ssh_socket s, const char *command);
 #endif
 void ssh_socket_close(ssh_socket s);