gtk2: errors in unixprint

[Jeff Johnson]

While investigating the gtk3 version of unixprint, it turns out there are
a couple of errors in the gtk2 version.

In the definition of _GtkPrinterClass, one element is a FuncPtr, but
does not have details correctly spelled out:

    struct _GtkPrinterClass {
        GObjectClass parent_class;
        void (*details_acquired) (void);

Should be (to match upstream):
        void (*details_acquired) (GtkPrinter *printer, gboolean success);

In the definition of GtkPrintCapabilities, the enumerator values are not
specified and so take the defaults, 0-9, while upstream declares them
as follows:

typedef enum {
  GTK_PRINT_CAPABILITY_PAGE_SET = 1 << 0,
  GTK_PRINT_CAPABILITY_COPIES = 1 << 1,
  GTK_PRINT_CAPABILITY_COLLATE = 1 << 2,
  GTK_PRINT_CAPABILITY_REVERSE = 1 << 3,
  GTK_PRINT_CAPABILITY_SCALE = 1 << 4,
  GTK_PRINT_CAPABILITY_GENERATE_PDF = 1 << 5,
  GTK_PRINT_CAPABILITY_GENERATE_PS = 1 << 6,
  GTK_PRINT_CAPABILITY_PREVIEW = 1 << 7,
  GTK_PRINT_CAPABILITY_NUMBER_UP = 1 << 8,
  GTK_PRINT_CAPABILITY_NUMBER_UP_LAYOUT = 1 << 9
} GtkPrintCapabilities;