New option: DEBUGLOCKS

[Constantine Sapuntzakis <csapuntz_(_at_)_stanford_(_dot_)_edu>]

I've added the code into OpenBSD-curren to allow us to
detect certain violations of the vnode locking discipline. Once detected,
the kernel panics at the point of the violation, indicating the operation
and argument that were violating the discipline. If kernel DDB is enabled,
a back trace can also be taken at the point of the panic by typing trace. 

Violations of the locking discipline can lead to subtle race conditions which 
manifest themselves in instability and decreased up-time. 

I have not enable this code by default, for fear that it will cause
undue pain to many users. However, if you have time and non-critical machines,
I'd appreciate it if you added

option	DEBUGLOCKS

to your kernel configuration and reported any panics from the resulting
kernel.

Fixes for the panics will also be accepted. :)

-Costa