RHEL6 is shipped with libc 2.12 where no prlimit
helper provided (which was pushed into libc itself
since 2.13). So we implement own proxy for convenience.

Note, if libc does support prlimit itself we simply
don't use any proxy and call it directly.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>