Split <sys/debug.h> header

Split <sys/debug.h> header

To avoid symbol conflicts with dependent packages the debug
header must be split in to several parts.  The <sys/debug.h>
header now only contains the Solaris macro's such as ASSERT
and VERIFY.  The spl-debug.h header contain the spl specific
debugging infrastructure and should be included by any package
which needs to use the spl logging.  Finally the spl-trace.h
header contains internal data structures only used for the log
facility and should not be included by anythign by spl-debug.c.

This way dependent packages can include the standard Solaris
headers without picking up any SPL debug macros.  However, if
the dependant package want to integrate with the SPL debugging
subsystem they can then explicitly include spl-debug.h.

Along with this change I have dropped the CHECK_STACK macros
because the upstream Linux kernel now has much better stack
depth checking built in and we don't need this complexity.

Additionally SBUG has been replaced with PANIC and provided as
part of the Solaris macro set.  While the Solaris version is
really panic() that conflicts with the Linux kernel so we'll
just have to make due to PANIC.  It should rarely be called
directly, the prefered usage would be an ASSERT or VERIFY.

There's lots of change here but this cleanup was overdue.