libdeng2: Fixed crash when doing log output during atexit()

The log buffer has already been destroyed when atexit calls its
callbacks.
Fixed|Updater: Default location for downloads, better script for Mac OS X

The default location for Updater downloads is now the user's cache
location.

The autoupdater script for Mac OS X was improved so that it activates
the Installer application and automatically ejects the disk afterwards.