Octave: remove allocation of new octave_value in SWIG_Octave_SetGlobalValue()

- this introduces a memory leak, which becomes significant for large
  modules (many global variables) and many module re-loadings (e.g.
  during a long-running script)
- the original motivation was to prevent double-frees on exit, but this
  problem appears to have been fixed by the _Exit() hack in later commits,
  and in any case is an issue only for Octave ~3.2, so it should be safe to
  remove; tested by running Octave examples/test suite with Debian 3.2.4 and
  built-from-source 3.2.4, 3.4.3, and 3.6.3
Octave: install functions only once, when creating module

- once installed, Octave functions can never really be uninstalled
  (clear -f doesn't prevent the function being called again), so
  it makes no sense to install functions more than once
- this can lead to a significant speed-up of module loading times,
  up to a factor of 10 for a large module loaded multiple times
Update changelog for the previous two Octave patches