Minor changes in output, don't verbose-extract the tarball.

Also suppresses all the redirection-messages printed by wget when using the
sourceforge download system.
Let the user choose the system account names.

refs #97
Use long options to improve readability.
Minor readability improvements.
Use "mktemp" for creating temporary files and clean up afterwards.

The "mktemp" tool is part of the "coreutils" package in Debian since version
7.4-1 (June 2009), so it is available on all relevant platforms.

refs #97
Show a prompt sign when asking for user input.

refs #97