util: add a function for executing an extrenal tools (v2)

For executing an external tools we need to block a SIGCHLD
and to juggle file descriptors.

SIGCHLD is blocked for getting an exit code.

A problem with file descriptors can be if we want to set 2 to STDIN,
1 to STDERR, 0 to STDOUT for example.

v2: use helpers reopen_fd_as and move_img_fd

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