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)