seize: Simplify creds checkers

Don't do comparisons of creds insize core seizing routine. Instead,
pull the creds on the caller's stack and compare them there if
required.

At the same time more the proc_status_creds_dumpable() to the place
where it's only needed and make it static.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>