We cannot remove the _+_ trick, until old make is completely deprecated.
But we don't want to set it to + for bmake since it breaks make -N
which is used to supress the normal handling of targets marked with .MAKE
(which seems broken in fmake and might be why _+_ was introduced).
Add some comments to explain what's gong on.

Reviewed by:	obrien
Flag recursive make targets with .MAKE (has no effect on fmake)
make -n will still exectute such targets
make -N will not.

Reviewed by:	obrien
Flag recursive make targets with .MAKE
so that job token pipe is passed to them.
To avoid surprising anyone, only add .MAKE to ${TGTS} when -n
has not been specified (at least for Makefile).

Reviewed by:	obrien
Improve debugger visibility into queuing functions by removing the macro
scheme for defining inline command queuing functions.

Prefer enums to #defines.

sys/dev/xen/blkfront/block.h
	Replace inline function generation performed by the
	XBDQ_COMMAND_QUEUE() macro with single instances of each
	inline function (init, enqueue, dequeue, remove).  This was
	made possible by using queue indexes instead of bit flags
	in the command structure, and passing the index enum as
	an argument to the functions.

	Improve panic/assert messages in the queue functions.

	Combine queue data and stats into a single data structure
	and declare an array of them instead of each queue individually.

	Convert command flags, softc state, and softc flags to enums.

sys/dev/xen/blkfront/blkfront.c
	Mechanical adjustments for new queue api.

Sponsored by:	Spectra Logic Corporation
MFC after:	1 week