If a debug class was explicitly set to zero the debug system would not

If a debug class was explicitly set to zero the debug system would not
recognise it as there was no distinction made between zeroing a debug
class and just not setting it to anything.  I've added a
debuglevel_isset array in parallel with the debuglevel_class array to
fix this.

Added a couple of new debug classes which I might start filling out
to get smb, rpc header and rpc marshall/unmarshalling debugs tidied
up.

Fixed a bunch of cut&paste bugs in include/debug.h

Modified smbcontrol and the messaging system debug handler to like the
debuglevel_isset stuff.