support any boot drive

[stsp]

freedos (and fdpp) takes fdconfig.sys from
C: even when boots from other drive.
Same is for command.com, unless altered
with shell=.
In the source code I don't see C: being
hard-coded, so I think this is a bug, not a
decision.
Whatever it is, we need to properly support
the boot drive.

Andrew, do you want to take a look at that?
Currently dosemu2 boots fdpp automatically
w/o any extra setup, so this is very easy to
reproduce.

[stsp]

So the first real advantage over freedos:
fdpp now support any boot drive and freedos
only supports C:. :)