util: move open_proc_fd to service_fd

We need this to avoid conflicts with file descriptors,
which has to be restored.

Currently open_proc_pid() doesn't used during restoring
file descriptors, but we are going to use it to restore
sockets in proper network namespaces.

Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>