Add a debug directory under src/test, where we will house all of our debug

Add a debug directory under src/test, where we will house all of our debug
scripts that will aid the debugging of the kernel and other code.

I have added a GDB macro set in file 'gdb.kernel' which contains some
helpful macros, such as listing processes, iterating CPU's globaldata,
a kldstat(1) style macro etc.

The 'gdb.kernel' file was written by:

	Hiten Pandya <hmp@backplane.com>,
	Matthew Dillon <dillon@apollo.backplane.com>

The kldstat(1) macro should be helpful with the asf(1) utility which
is used for setting the right offset to load the KLD's symbols.