Improve ccache formatting.
    
    Add %U support which will insert the user uid number instead of name.
    Fix %% support by actually removing one of the % charcters
    Fix %<invalid> sequence by actually bailing out if one is found.
    Add GPDEBUG statements to indicate what went wrong.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>