Attempting to compile strict C99 user land applications or
libraries with the Lustre UAPI headers will fail. These same
errors can be seen by enabling CONFIG_UAPI_HEADER_TEST as well.
Update the Lustre UAPI headers to be compilable with -std=c99.
Enhance our current test covering UAPI header handling.

For OpenSFS branch we can't include <linux/stat.h> since we support
kernels before struct statx existed and they will collide with the
special definitions in lustre_user.h.

Change-Id: Ifb0da33180dc3c7e116d6bf2b7f603ad0528277a
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/37678


Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>