Parasite daemon mode might be considered as a series
of network messages sent from main crtools module to
parasite daemon which spins on a socket waiting for
command to handle.

Simple command handling session might look as

        crtools                     daemon
        |                           |                           |
        | ---> command message ---> |                           |
        |                           | ---> receive command ---> |
        |                           |           ....            |
        |                           |       handle command      |
        |                           |           ....            |
        |                           | <--- send ack       <---  |
        | <--- receive ack     <--- |                           |
        |                           |                           |
        v                           v                           v
      time

where, in case of multithread application [handle command] stage
includes waking up of children threads via futexes, wait until
handling complete and only then send ack back to ctrools module.

This patch brings in description of command message as

struct ctl_msg {
    unsigned int    id;     /* command recipient */
    unsigned int    cmd;    /* command itself */
    unsigned int    ack;    /* ack on command */
    int             err;    /* error code on reply */
};

For example, in case of dumpee being with pid 2 and acquiring
to handle PARASITE_CMD_DUMP_MISC, the command/ack series will look
as

    cmd-msg
    -------
    .id     = 2,
    .cmd    = PARASITE_CMD_DUMP_MISC,
    .ack    = 0,
    .err    = 0,

    ack-msg
    -------
    .id     = 2,
    .cmd    = PARASITE_CMD_DUMP_MISC,
    .ack    = PARASITE_CMD_DUMP_MISC,
    .err    = 0

in case if error happened in parasite, the @err field set accordingly.

For convenience ctl_msg_cmd()/ctl_msg_ack() macros are provided
for control message generations.

v2: delete futex_t ack from tid_state_s
Fixed-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>