parasite: Rename gen_parasite_saddr @pid to @key

This routine will be used more widely and we will
be generating sockets with name based on some predefined
unique value rather than 'pid', so name argument this
way.

No change on functional level.

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