unix: add ability to set callbacks for external sockets (v5)

We don't know a state behind an external socket. It depends on logic
of the program, which handles this socket.

This patch adds ability to load a library with callbacks for dumping
and restoring external sockets.

This patch introduces two callbacks cr_plugin_dump_unix_sk and
cr_plugin_restore_unix_sk. If a callback can not handle a socket, it
must return -ENOTSUP.

The main questions, what kind of information should be tranfered in
these callbacks. Pls, think a few minutes about that and send me
your opinion.

v2: Use uflags instread of adding a new field
v3: clean up
v4: Unsuitable callbacks return -ENOTSUP.
v5: set USK_CALLBACK, if a socket was dumped by callback.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/plugin.h

@@ -7,4 +7,8 @@
 
 void cr_plugin_fini(void);
 int cr_plugin_init(void);
+
+int cr_plugin_dump_unix_sk(int fd, int id);
+int cr_plugin_restore_unix_sk(int id);
+
 #endif