scm: Unweave fds mess

Currently criu built with criu/pie-util-fd (which
is a symlink to criu/pie/util-fd) with same flags
as we use in general compel infection code. Moreover
the criu link with libcompel.a, so we get a problem
where send_fds/recv_fds are multiple defined. Lets
rather unweave this mess:

 - drop criu/pie-util-fd.c completely
 - move send_fd/recv_fd inliners into scm.h

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>