A bunch of fixes for cjdns.sh

Fixed cjdns.sh to properly test for the existence of /etc/default/cjdns (the previous way may have worked on some systems, but it didn't on mine, and this new way should work for everyone). The start function now tests to see if cjdns is running before it attempts to start it, as the previous method caused it to hang for me when cjdns was already running (the output now explicitly states whether the error was that cjdns was already running or some other unknown issue too). The update function won't work in setups that don't have a cjdns source directory, so it now tests to see if the .git folder exists before attempting to run the update.