Generate changes entries from special files

If the new flag 'changesgenerate' is set, tarballs are searched for
files such as CHANGES(.txt), NEWS or ChangeLog. A diff is then generated
between the old and new version of this file and written to the
respective changes file.

Can be customized further by adjusting:

  + changesauthor (does the right thing by default)
  + changeslinesmax (30 by default)