tsort: Error out if writing to stdout failed.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D39959

(cherry picked from commit cb46f47c7969b619d1b2547d06a75ad5b375eda9)

tsort: Replace bcopy() with memcpy().

Also fix an indentation error I introduced in the previous commit.

Fixes: cb46f47c7969
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40042

(cherry picked from commit b55bc49e8694d9226a82041ff23ad61a5c7a6a76)

tsort: Add unit tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40043

(cherry picked from commit 09aee570980b7eca6e3c902a66f6db129b8c7376)