diff: add support for --help and --version

[thj]

Summary

Add support for --help and --version to be compatible with gnu diff.

gnu diff --help writes to stdout, do the same to be compatible

Test Plan

Test suite was run and no failures found

[debdrup]

A tiny nit with the mdoc syntax, otherwise it looks good to me.

usr.bin/diff/diff.1

316

A macro missed its stop on the chair-keyboard bus. ;)

[pauamma_gundo.com]

usr.bin/diff/diff.1

289

Worth mentioning "to standard output"? I'm on the fence on this.

316

A macro missed its stop on the chair-keyboard bus. ;)

Or maybe 2 collided causing a wreck. gdiff(1) says "-v, --version"

317

Same question as for --help.

usr.bin/diff/diff.c

56

For full compatibility, also needs to support -v as a --version synonym here and *gestures vaguely toward the switch statement below*

[thj]

Update based on man page feedback

[pstef]

I can only see the man-page change, should I be seeing code changes in this diff as well?

[thj]

Add back the code changes

[thj]

Yeah, wrong diff point with arc diff, I am sure arcanist wasn't this much trouble to use before

[pauamma_gundo.com]

Is there a reason for adding only --version and not the short flag synonym -v?