rc.d/sendmail: Fix stopping service during shutdown

The sendmail service script needs to be stopped during shutdown
to ensure a clean shutdown of active SMTP connections (and writing
any in memory queue files).

rcorder(8) requires the rcorder block to be an uninterrupted sequence of
REQUIRE, PROVIDE, BEFORE, and KEYWORD lines. Having a comment in between
REQUIRE and KEYWORD makes rcorder stop parsing the block when it reaches
the comment.

Fix that by moving the comment out from the rcorder block.

Reviewed by: bnovkov, christos, gshapiro, markj
Approved by: bnovkov (mentor), christos (mentor), markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46924

(cherry picked from commit 8751fbe36ff02ed695f02132ee6eac723d2bbe3f)