Changes to ramdisk driver to support disklabels. Work done by Theo.

Changes to ramdisk driver to support disklabels. Work done by Theo.
This will require changes for each machine type.
changes to MAKEDEV, suggest treating rd like sd
---
sd*|vnd*|ccd*|rd*)
...
	rd*)  name=rd;  blk=17; chr=17;;
---
Make certain that majors and minors are correct
conf.c
	add a entry in cdevsw for rd (previously was only in bdev)
---
cdev_decl(rd);
...
struct cdevsw cdevsw[] = {
	...
	cdev_disk_init(NRD,rd),         /* 17 ram disk driver*/
---

other changes to how rdconfig is called may be appropriate,
it must now be configured using the "raw" parition typically 'c'.