[CRIU] [PATCH 0/5 RFC] add ability to set callbacks for external sockets (v2)

[Andrey Vagin]

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

The patch adds ability to load a library with callbacks for dumping
and restoring external sockets. In a future we can add callbacks
for other external object.

A library can have:
* two functions cr_init and cr_fini, which are called
  on loading and unloading.
* two callbacks cr_dump_unix_sk and cr_restore_unix_sk for dumping and
  restoring unix sockets. If a callback can not handle a socket, it
  must return CR_SK_SKIP.

v2: * Scan libraries in a predefined directory. The path cab be set by
      the -L option or the CRIU_LIBS_DIR environment
    * Look up all callbacks on loading of libraries
    * The test uses two libraries
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1.8.3.1