Allow 'build' and 'install' to run separately again, issue #49

This was tricky to fix, since I wanted to preserve the ability to
use "rumpmake install" to install components into destdir.  So what
I ended up doing was creating two different make wrappers, one run
inside buildrump.sh (_buildrumpsh_rumpmake) and one meant to be run
by users afterwards (rumpmake).  Internally, during the build stage,
buildrump.sh installs everything into a staging directory in objdir.
The install stage now uses tar to copy the staging directory into the
dest directory.  The user-targeted rumpmake always install into the real
destdir (i.e.  what tar copies the files into).  As an added bonus, the
"build" stage for buildrump.sh no longer ever depends on elevated privs,
since includes too are installed into objdir during "build".

Finally, I removed the flattening of the man hierarchy, since most
systems seem to expect share/man/man<n>/cat<n>.  And by "most" I
mean at least one Linux distro.