There are lots of files that don't need it: The number of object
files that actually need it went down from 2011 to 884 here.
Keep it for external users in order to not cause breakages.
Also improve the other headers a bit while just at it.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
This was introduced in version 4.6. And may not exist all without an
optional package. So to prevent a hard dependency on needing the Linux
kernel headers to compile, make this optional.
Also ignore the status of the ioctl, since it may fail on older kernels
which don't support it. It's okay to ignore as its not fatal and any
serious errors will be caught later by the mmap call.
