accounting: explicitly mark the exiting thread as doing accounting

and use the mark to stop applying file size limits on the write of
the accounting record. This allows to remove hack to clear process
limits in acct_process(), and avoids the bug with the clearing being
ineffective because limits are also cached in the thread structure.

Reported and reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30257