Update diff(1) TODO removing what has been implemented

diff(1): Implement -B/--ignore-blank-lines

As noted by cem in r338035, coccinelle invokes diff(1) with the -B flag.
This was not previously implemented here, so one was forced to create a link
for GNU diff to /usr/local/bin/diff

Implement the -B flag and add some primitive tests for it. It is implemented
in the same fashion that -I is implemented; each chunk's lines are scanned,
and if a non-blank line is encountered then the chunk will be output.
Otherwise, it's skipped.

MFC after: 2 weeks

Update the TODO list to reflect what has been changed

Import diff from OpenBSD and remove GNU diff

Some of the modifications from the previous summer of code has been integrated
Modification for compatibility with GNU diff output has been added

Main difference with OpenBSD:
Implement multiple GNU diff options:
* --ignore-file-name-case
* --no-ignore-file-name-case
* --normal
* --tabsize
* --strip-trailing-cr
Make diff -p compatible with GNU diff
Implement diff -l
Make diff -r compatible with GNU diff

Capsicumize diffing 2 regular files
Add a simple test suite

Approved by: AsiaBSDcon devsummit
Obtained from: OpenBSD, GSoC
Relnotes: yes