crtools: close a signal descriptor after passing a preparation stage

This patch adds the --siganl-fd FD option to specify a file descriptor.
CRIU will write '\0' to this descriptor and close it after passing
a preparation stage.

It is alternative way to demonizing a criu process after a preparation
stage. It's imposiable to get exit code, if a process has daemonized.

The introduced way allows to wait a preparation stage and to get an exit
code. It can be easy used from shell and other script languages.

v3: fix a help message

v4: Here is a sequence of actions how it can be used:
     * open a pipe
     * run a service with the pipe[1] as status_fd
     * read(pipe[0]) to wait a moment when the service will be ready to
       accept connections
     * do a work which requires the service
     * wait the service process to gets its exit status to be sure that
       everything okey

travis-ci: success for crtools: close a signal descriptor after passing a preparation stage (rev6)
Cc: Mike Rapoport <mike.rapoport@gmail.com>
Cc: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>