GEOM_DEBUG kernel option

[Dmitry Luhtionov]

I introduce new kernel option GEOM_DEBUG
Its enabled on default, but disable it reduce runtime kernel size
and slightly improve perfomance by disabling debug stuff in compile time

On my amd64 machine compiling modules without GEOM_DEBUG option reduce size
geom_mirror.ko from 130 to 77 kilobytes
geom_label.ko from 44.5 to 33 kilobytes

http://193.34.20.243/geom.20100611.patch