auto merge of #13897 : aturon/rust/issue-6085, r=bjz

The `std::bitflags::bitflags!` macro did not provide support for
adding attributes to the generates structure, due to limitations in
the parser for macros. This patch works around the parser limitations
by requiring a `flags` keyword in the `bitflags!` invocations:

    bitflags!(
        #[deriving(Hash)]
        #[doc="Three flags"]
        flags Flags: u32 {
            FlagA       = 0x00000001,
            FlagB       = 0x00000010,
            FlagC       = 0x00000100
        }
    )

The intent of `std::bitflags` is to allow building type-safe wrappers
around C-style flags APIs. But in addition to construction these flags
from the Rust side, we need a way to convert them from the C
side. This patch adds a `from_bits` function, which is unsafe since
the bits in question may not represent a valid combination of flags.

Finally, this patch changes `std::io::FilePermissions` from an exposed
`u32` representation to a typesafe representation (that only allows valid
flag combinations) using the `std::bitflags`.

Closes #6085.

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