fix log_filter, remove now superfluous SAGA_PTYDEBUG

Hi Ole,

to disable the PTY log messages, please set

   export SAGA_LOG_FILTER='!PTYShell'

in your env, and they will disappear.
remove dead code
cosmetics on shell job management

One of the failures reported was caused by the recently introduced job purge --
I fixed the tests to cater for that.  I am not yet sure what is up with the
lonestar prolems reported by melissa - need more tests
Merge branch 'fixes' of github.com:saga-project/saga-python into fixes