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

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

Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

When passingthrough from sdiff to diff the -H/--speed-large-files
options rename it to the long version as GNU diff only support the long
version of the option not the short version

Remove useless .Pp after the .Sh macro and remove empty line

Reported by: make manlint
MFC after: 2 days

Revert r299218 VISUAL is actually a perfecly valid env to specify an editor

Only use EDITOR as a variable to specify which text editor to use

import sdiff(1) from GSoC 2012

Import sdiff(1) from the diff version written by Raymond Lai,
improved during GSoC 2012 by Jesse Hagewood.

Compared to the version done in during that summer of code:
- Remove the zlib frontend: zsdiff
- Compatible output (column size and separators) with GNU sdiff

Compared to GNU sdiff in ports:
- The only difference is padding using spaces vs tabs

Compared to OpenBSD and NetBSD import:
- Implement missing options (including long options) from GNU sdiff
- Improved support for the edition mode (signal handling)
- Output visually compatible with GNU sdiff: size of columns

While here import regression tests from NetBSD adapted to fit the output as
expected by GNU sdiff

Reviewed by: emaste (in part)
Obtained from: OpenBSD, NetBSD, GSoC 2012
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D5981
Differential Revision: https://reviews.freebsd.org/D6032 (diff with NetBSD version)
Differential Revision: https://reviews.freebsd.org/D6033 (diff with OpenBSD version)