Currently we prepare a parasite socket only once and
save it in a static variable.

It's bad idea to use a static variable in a library.

In addition, it doesn't work if we have processes in
different network namespaces. In this case, we have to have
a separate socket for each namespace.

v2: fix compilation on Alpine
    convert *p_sock into sock

travis-ci: success for compel: check whether a parasite socket is prepared each time (rev2)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>