vzmigrate: use C locale

As reported in bug #3049, printf fails to print floating-point
numbers when locale is set to one that uses comma delimiter for floats.

Fix this (and possibly other issues with locale) by setting it to C
(the code is stolen from autoconf).

https://bugzilla.openvz.org/3049

Reported-by: Pavel Odintsov <pavel.odintsov@gmail.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>