Require and document using kernel headers from "make headers_install"

[Roland McGrath]

> Thanks.  As suggested for the "GNU system" variants I propose creating 
> Texinfo macros @theglibc{} ("the GNU C Library") and @Theglibc{} ("The GNU 
> C Library") and making all references use one of those macros (except 
> maybe where giving the name of the manual itself, since there I think 
> "The" logically binds to "Reference Manual" not "GNU C Library").

Yes, I was going to suggest that if you didn't.