Upgrade to Bitflags 1.0

[Susurrus]

This isn't straightforward because something about 944ee530cd04712021ee21a60f1ef59d2ab6d489 broke our libc_bitflags! macro with the following error:

   Compiling bitflags v0.9.1
   Compiling nix v0.10.0-pre
error: no rules expected the token `cfg(any(target_os = "android", target_os = "linux"))`
   --> src/macros.rs:129:19
    |
129 |                   #[$attr]
    |                     ^^^^^
    | 
   ::: src/fcntl.rs
    |
11  | / libc_bitflags!{
12  | |     pub struct AtFlags: c_int {
13  | |         AT_SYMLINK_NOFOLLOW;
14  | |         #[cfg(any(target_os = "android", target_os = "linux"))]
...   |
18  | |     }
19  | | }
    | |_- in this macro invocation

error: Could not compile `nix`.

To learn more, run the command again with --verbose.

So that should get figured out so we get nice constant scoping in our docs. Note that this will require use to require 1.20 as our minimum Rust version, but I think it's worth it for us to switch to that because of the significant documentation win.