MFC r252415, r252422:

acct: reduce code duplication by using acct_disable as cleanup for
failed kproc_create

acct: create a special plimit object and set it for exiting processe
instead of allocating new one each time

All limits are set to RLIM_INFINITY which sould be ok (even though we
care only about RLIMT_FSIZE in this case).