MFC r351192, r351203

r351192:
periodic: fix anticongestion for scripts run after security

Revision 316342, which introduced the anticongestion feature, failed to
consider that the periodic scripts are executed by a recursive invocation of
periodic. The recursive invocation wrongly cleaned up a temporary file that
should've been cleaned up only by the original invocation. The result is
that if the first script that requests an anticongestion sleep runs after
the security scripts, the sleep won't happen.

Fix this bug by delaying cleanup until the end of the original invocation.

PR: 236564
Submitted by: Yasuhiro KIMURA <yasu@utahime.org>
Reviewed by: imp

r351203:
periodic: replace "tty" with "test -t 0"

Apparently using tty for this purpose has been deprecated since 4.4 Lite.

Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D21318