style(9) rules for nested includes

[mdf at FreeBSD.org]

I recall a recent discussion/PR about nested includes in the context
of <sys/linker_set.h> and <sys/queue.h> being a few of the only ones
allowed.  However, I don't see anything in style(9) about this.

One rule for nested includes I've heard (that FreeBSD doesn't use, but
just mentioning it for variety) is that every header file should
compile on its own; that is, a .c file with nothing but #include
<header.h> should compile without errors.  This rule also makes some
sense with the FreeBSD style convention of alphabetizing includes;
otherwise it's kinda just happenstance that e.g. <sys/lock.h> comes
alphabetically before <sys/sx.h>, <sys/mutex.h>, <sys/rwlock.h> and
<sys/rmlock.h>, and barely before <sys/lockmgr.h>.

Now we come to the reason I ask.  I'm working on a patch to change the
static sysctl code to use the standard SYSININT/SYSUNINIT code rather
than have special treatment in kern_linker.c, but to do this I need to
either change quite a few places that include <sys/sysctl.h>, or
include <sys/kernel.h> instead of <sys/linker_set.h> in sysctl.h, as
the SI_SUB_SYSCTLS value isn't visible otherwise.

This, though, shows a bug in <sys/kernel.h>, where it uses the
constant MAXPATHLEN in the extern declaration of kernelname[] (which
itself I think is not standard C, IIRC, but there are several uses of
sizeof(kernelname) that don't work otherwise).  So then <sys/kernel.h>
needs <sys/param.h> to build.

So, which files are okay to include in an include file?  Does someone
(I'll do it if we can agree on a list) want to put this in style(9)?

Thanks,
matthew