small enhancement idea for 'make installworld' et al.

[David Rhodus]

around that.  I don't personally see how having 'su root' in the
Makefile is less safe than making the user manually type 'su root' or
making them re-login as root - in fact, one could make the argument 
that
it's safer: while the human could forget to exit the su back to a 
normal
user when the installworld is done, the Makefile does that part
automatically too.
I don't see that this takes into account setting DESTDIR at all.

Typing `su root` is not safe at all. To make sure unsafe environment
settings aren't pulled over from the user account your are switching
from you must type `su - root`.
I think a warning message would be fine here if the user that
just typed installworld wasn't root. But the Makefile doesn't
need to take any action though based on that.
-DR