Extended load_fd() to support reading from the first Primary DOS

Extended load_fd() to support reading from the first Primary DOS
partition of the boot disk.  So we have yet another medium via
which to load the FreeBSD distribution files.  load_fd() has
options for listing and (if reading from the C:  drive) changing
directories.

load_fd's notation assumes that the first Primary partition on
disk is the DOS drive C: (since this and only this one is mounted
by install).  Otherwise, the notation may be a bit confusing.
We'll know the assumption is bad if people complain about
not finding files on their "C:" drive...

Added a device file existence check to kc.profile.