checkpatch: adjust to QEMUisms

Change checkpatch.pl for QEMU use:
 - Root directory detection
 - Forbid tabs
 - Indent at 4 spaces
 - Allow typedefs
 - Enforce brace use even for single statement blocks
 - Don't suggest nonexistent cleanup tools

Mention the script in CODING_STYLE.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
