creds: dump -- Implement per-thread dump of credentials

This as well as restore requires several steps to reach per-thread
support during dump stage

 - @creds area to be fetched from the parasite is embedded into
   parasite_dump_structure

 - when test for task to be dumpable we no longer compare caps
   because we now allow them to be different (and I renamed
   proc_status_creds_eq to proc_status_creds_dumpable for this
   sake)

 - have to extend dump_thread_common to support dumping of
   creds (we call for dump_thread_common in several places,
   in particular when we need to fetch misc params we don't
   need creds, here @creds option comes into the play)

 - after this patch no creds-X.img file be generated anymore,
   I guess we might drop it off with time from descriptors

https://jira.sw.ru/browse/PSBM-41416



v2:
 - In dump_task_creds() don't mangle the call for parasite_dump_creds
   and collect_lsm_profile
 - PARASITE_MAX_GROUPS takes parasite_dump_thread into account because
   dump_thread_common now serves two cases: for plain misc parameters
   fetching and for creds as well (depending on the context)
 - when test for dumpable we still require the seccomp filters
   to match, they can be different and we need to support such
   configuration too but not in this series

v3:
 - Rip off dump_task_creds completely, together with PARASITE_CMD_DUMP_CREDS,
   we dump creds unconditionally in dump_thread_common
 - the group leader thread data is fetched via new
   parasite_dump_thread_leader_seized helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>