enforcing page color matches

[Matt Thomas <matt%3am-software.com@localhost>]

I modified the mips pmap to drop into ddb if there was an attempt to map a page 
with an incorrect page color (color(VA) != color(PA)).  Then using UVMHIST I 
fixed each cause.  I can now boot a kernel on my matt-nb5-mips64 branch into 
sysinst with no bad color matches.  I'm sure I haven't found all the cases but 
it's a big step forward.

The diffs are relatively minor and are at 
www.netbsd.org/~matt/uvm-color-diffs.txt for your viewing enjoyment.  

I'm using a MIPS 74K which needs strict page-coloring enforcement (4 colors for 
its Icache and 2 colors for its Dcache) so this is important to me.  If this 
can be enforced, the code to deal with bad colors can be removed and that will 
greatly simplify the mips pmap code.