Add a simple RAM disk driver

* It uses physical pages directly, not wasting kernel address space.
* The user interface is somewhat crude, it uses writes to a control
device (output to serial debug/syslog), e.g.
echo register 4g > /dev/disk/virtual/ram/control
to register a 4 GB RAM disk. It is published under
/dev/disk/virtual/ram/<index>/raw and can be formatted with DriveSetup
like any other disk. Unregistering is broken ATM.
* It's not on the image by default, but can be added via
AddNewDriversToHaikuImage disk virtual : ram_disk ;
* I found it quite useful when working on large HaikuPorts ports.
E.g. mounting a RAM disk at the port's work directory shaves a lot of
time off disk heavy operations. Obviously one shouldn't let changes
lie around on it without back up.

Add a simple RAM disk driver

* It uses physical pages directly, not wasting kernel address space.
* The user interface is somewhat crude, it uses writes to a control
device (output to serial debug/syslog), e.g.
echo register 4g > /dev/disk/virtual/ram/control
to register a 4 GB RAM disk. It is published under
/dev/disk/virtual/ram/<index>/raw and can be formatted with DriveSetup
like any other disk. Unregistering is broken ATM.
* It's not on the image by default, but can be added via
AddNewDriversToHaikuImage disk virtual : ram_disk ;
* I found it quite useful when working on large HaikuPorts ports.
E.g. mounting a RAM disk at the port's work directory shaves a lot of
time off disk heavy operations. Obviously one shouldn't let changes
lie around on it without back up.