The original argument for blocking DMA 0 was to avoid conflicts with
"memory refresh"  but such configurations are only found on very old
8-bit systems that are likely not supported by the linux kernel. 
This patch allows dma 0 to be assigned to PnP devices by default.  If
for whatever reason dma 0 cannot be used, one can avoid allocating it
by setting the pnp_reserve_dma= kernel parameter.