i2c-tools: Implement and document option -h

Add an option -h to display the help of the tools. This is a common
practice so users may expect it to work.

Based on a preliminary patch from David Kerns.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
10 files changed