I think that this was done on a one-to-one basis, and never consistantly.
With the advent of `configure' as the userland check for features needed
by a package to compile, one does not get to see the compilation errors
anymore and it is difficult to determine what is going on. Also one cannot
predict what header files a userland program will need. Eg. amd uses
<ufs/ufs/ufsmount.h>. Since we install those header files in /usr/include,
it is conceivable that some userland program will include them. 

I propose that we make the rounds and protect all of them at this point
against double inclusion. I know about the arguments about efficiency
etc., but having userland programs not build correctly because we don't
do this is simply silly.

If I don't hear any arguments against, I'll start the process in a few
days.

christos