Added kernel debug build option

From the top makefile, you can now build the kernel with debugging support by
typing 'make debug' rather than 'make'. This uses a make variable called
DEBUG, which could also be passed on the command line with 'make DEBUG=y'.

Signed-off-by: Joel Nider <joel@nider.org>
JOEL4: leave system call parameters on stack

Trial implementation. Seems to work, but I still have some doubts about its
security. A more secure implementation may reduce efficiency. I will use it
for now, but will wait for some performance testing before making a final
judgement.

Signed-off-by: Joel Nider <joel@nider.org>