configure: enable --extra-cflags for Windows

Add configure help as well.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Fix crash with absurdly but not impossibly deeply nested device stacks

Use the proper PATH_MAX for the max path for disk util, and use
snprintf() to avoid potentially overwriting it.

Modified by Jens to return NULL instead of exiting.

Signed-off-by: Jens Axboe <axboe@kernel.dk>