In autogen.sh use piped input for ProcessOptionsFile
Minor changes to autogen.sh etc. help text
In autogen.sh etc use the default options file by default

If 'current' options file doesn't exist then we have to pick some file
to use or crash. Using the 'default' options file seems sensible.
In interactive_autogen.sh correctly handle the case where current does
not exist
In interactive_autogen.sh sort the list of options files