gh-107538: [Enum] fix handling of inverted/negative values (GH-132273)

* Fix flag mask inversion when unnamed flags exist.

For example:

    class Flag(enum.Flag):
        A = 0x01
        B = 0x02
        MASK = 0xff

    ~Flag.MASK is Flag(0)

* EJECT and KEEP flags (IntEnum is KEEP) use direct value.

* correct Flag inversion to only flip flag bits

IntFlag will flip all bits -- this only makes a difference in flag sets with
missing values.

* correct negative assigned values in flags

negative values are no longer used as-is, but become inverted; i.e.

    class Y(self.enum_type):
        A = auto()
        B = auto()
        C = ~A        # aka ~1 aka 0b1 110 (from enum.bin()) aka 6
        D = auto()

    assert Y.C. is Y.B|Y.D