Clean up fallout proc_is_traced_p() change:
- proc_is_traced_p() -> trace_is_enabled(), to match trace_enter() and
  trace_exit().
- trace_is_enabled() becomes a real function.
- Remove unnecessary include files from various files that used to care
  about KTRACE and SYSTRACE, but do no more.
Syscall debug tracing is handled by trace_enter() / trace_exit().  Change
trace_is_enabled() to return TRUE if SYSCALL_DEBUG is defined, and g/c
all of the SYSCALL_DEBUG handling from individual system call dispatch
routines.
Fix build problems caused by the last commit by only providing the
prototypes for trace_XXX() if _KERNEL is defined and including
"sys/types.h" in that case to get the definition of "boolean_t".
Remove another instance of the macro triplet SET/CLR/ISSET, now
to be found in <sys/types.h>.
remove the no longer useful fdavail(), as proposed and (thankfully) not
discussed on tech-kern
Add some information about the kernel authorization branch.
Add kernel authorization routines.
Adapt to kernel authorization KPI.
Compile kern/kern_auth.c too.
Adapt to kernel authorization KPI.
Adapt to kernel authorization changes.
Adapt to kernel authorization KPI.

I expect *some* lossage here...
Adapt to kernel authorization KPI.
Fix syntax error.

Excellent catch by Masao Uebayashi, thanks!
Regen for kernel authorization KPI.
Adapt to kernel authorization changes.
Adapt to kernel authorization KPI.

This could use some testing...
Adapt to kernel authorization KPI.
Adapt to kernel authorization KPI.
Adapt to kernel authorization KPI.