Enable DMA by default in kernel for (S)ATA controllers.

This is needed since some (S)ATA controllers doesn't work properly when
DMA is not activated. When this happens, it prevents to start fossil,
then DMA cannot be enabled from startup scripts. Also, it makes the
first startup much faster when fossil do its initial dump to venti.

An option "*nodma=" was added to plan9.ini to disable DMA if needed.
This option is set in the installation plan9.ini, since the user is
asked to enable or disable DMA at the beginning of the installation
process.

I considered it is preferable to enable DMA by default since most disk
controllers are SATA these days and DMA is part of the specification.
Also, most modern ATA controllers have been known to support DMA
properly.

Optionally, the bootsetup installation script could automatically add
option "*nodma=" in installed plan9.ini when the user choose to
disable DMA.