Add alignment feature and use #[repr(align(x))]
Trying to solve #1042.
Here I introduce the discussed feature that will allow going from struct alignment with a private `__align` field to using `#[repr(align(x))]`. However, I have not implemented it for all structs that require alignment yet, only `in6_addr`. This because I did not want to spend too much time before we have discussed and solved the remaining questions regarding this.
One thing to discuss is testing. I have so far not done anything to the CI scripts. So currently they will still test the crate only with the `align` feature disabled. Thus they will make sure the `__align` fields are still correct. But no automatic tests make sure everything is correct when the `align` feature is turned on. What do we want to do about that? Can we insert another `cargo test` with `--features align` to make all the Travis jobs run the test suite twice, or will that slow things down too much?
I have tried using this version of libc in rustc and the standard library. And successfully changed `Ipv6Addr::new` to not use any `unsafe` and to become a `const fn`. Whether or not we want that is out of scope for this PR, but my point was that the changes introduced with this PR allow much more flexible usage of the libc structs that have alignment.
Openbsd fix
put in several fixes for OpenBSD (one fix per commit).
testsuite ran on OpenBSD 6.3-current (upcoming 6.4)
```
RUNNING ALL TESTS
PASSED 6474 tests
```
Bump ctest from `482c7f0` to `5c53723`
Bumps [ctest](https://github.com/alexcrichton/ctest) from `482c7f0` to `5c53723`.
<details>
<summary>Commits</summary>
- [`5c53723`](5c537236d1) Update libc dep
- See full diff in [compare view](482c7f0643...5c537236d1)
</details>
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Add linux musl powerpc (32-bit) support
This change adds support for musl on 32-bit powerpc. Most of the general constants in src/unix/notbsd/linux/musl/b32/mod.rs were different from those of the powerpc port, so I moved them over to the individual architecture-specific submodules. The same with the ipc_perm structure.
Add ENOATTR for Android
PR based on https://github.com/rust-lang/libc/pull/594
It's defined in Android sysroot so it should work without test workaround (CI should catch it otherwise?).
