Allow installation of extracted packages with pkg.m

[Thomas Weber]

The attached changeset allows the installation of already extracted
tarballs. It hooks into the normal installation process, just after the
extraction.

There's a FIXME comment in the patch, as I don't understand the
necessity for the 
  from k=3: length(dirlist)
loop. If you look at the unchanged code, length(dirlist) is checked to be not >3
just before this loop.

By the way, I think a dummy pkg inside the Octave sources for testing
pkg.m would make sense (to have meaningfull diffs, it would be tarred up
as part of the unit testing). 

        Thomas

pkg.diff
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