and add prereq targets, so some header files are generated by BUILDUSER
during 'make prereq' instead of by root during 'make includes'.
Switch the order of 'make cleandir' and 'make includes' during 'make build'
so we don't generate many files twice.
Except for some machine@ symlinks from ${MACHINE}/stand, /usr/obj is now
clean from files generated by root during 'make build'. Those will be
cleaned up in a second step.
help, testing & ok deraadt, input from natano, further testing rpe
tb