sdiff: Misc cleanup.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43943

(cherry picked from commit 3cc86989bfbe27c91b5db592c2af33fef153e230)

sdiff: Fix --expand-tabs and --tabsize.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43941

(cherry picked from commit a834edfccd14a8c0f152a3b0078469af8e05f3fd)

sdiff: Fix binary case.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43942

(cherry picked from commit ad7bef8b890768e68a48bbfa6b92ebf635068504)