libdeng2|FS: Improved interface for file creation

Instead of a simple bool flag, use an enum for the behavior when
creating new files.
libdeng2|App: Added -reset for emptying persist.pack to defaults

The new command line option "-reset" will delete persist.pack causing
all persistent data to be lost, effectively restoring the engine to
"factory defaults".

In the future this can be used to reset all console variables as well
(and all other persistent state) since they will be stored in the
Config.