parasite: Refactor code to follow calling convention

It happened some routines in parasite service code
were not following calling convention so I fixed the
callers and added a comment about adding new code here.

At the same time the 3 error code madness is dropped
as being requested by Pavel -- now we return one error
code only.

The PARASITE_ERR_ codes were dropped as well due to
become redundant.

The status of parasite service routine is set via
SET_PARASITE_RET helper. In case if there is no error --
just return 0. The calling code should not expect to
find anything sane in parasite_status_t because parasite
code might not touch it at all.

Same time, due to this convention the parasite's
dump_socket_queue is getting rid of third @err
member, because it's now returned as a regular
error code.

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