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.

Add some posix_spawn apple extension flags

Revert "Link to libgcc when statically linking musl"

This causes linker errors due to duplicated symbols. See https://github.com/rust-lang/rust/pull/52157

This reverts commit 920cfeac.

Add MODULE_INIT_ constants

These are flags required to implement the linux kernel loading mechanism.
Specifically finit_module.

These are flags required to implement the linux kernel loading mechanism.
Specifically finit_module.

Implement statfs for dragonfly, freebsd and openbsd

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
```

they doesn't exists on OpenBSD. put them under NetBSD specific tree.

Fix netbsd build with "stdbuild"

A bug was introduced in #930 which caused netbsd to not link to libm.

r? @alexcrichton

Fix a #[cfg] attribute

It looks like this was a mistake of #930 but should be easy to fix!

It looks like this was a mistake of #930 but should be easy to fix!

Link to libgcc when statically linking musl

On some architectures (e.g. aarch64) musl depends on some libgcc functions (`__addtf3`, `__multf3`, `__subtf3`) for `long double` arithmetic that it uses internally. Unfortunately we don't provide these functions in compiler-builtins, so we instead need to get them from libgcc.

Fixes:
https://github.com/rust-lang/rust/issues/46651
https://github.com/rust-lang-nursery/compiler-builtins/issues/201
https://github.com/rust-lang-nursery/compiler-builtins/issues/217

Bump ctest from `482c7f0` to `5c53723`

Bumps [ctest](https://github.com/alexcrichton/ctest) from `482c7f0` to `5c53723`.
<details>
<summary>Commits</summary>

- [`5c53723`](https://github.com/alexcrichton/ctest/commit/5c537236d150da311d67b8ccf4c3c49987b92c8d) Update libc dep
- See full diff in [compare view](https://github.com/alexcrichton/ctest/compare/482c7f0643942174a802d89ad7d460e89b576ed3...5c537236d150da311d67b8ccf4c3c49987b92c8d)
</details>
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Bumps [ctest](https://github.com/alexcrichton/ctest) from `482c7f0` to `5c53723`.
- [Release notes](https://github.com/alexcrichton/ctest/releases)
- [Commits](https://github.com/alexcrichton/ctest/compare/482c7f0643942174a802d89ad7d460e89b576ed3...5c537236d150da311d67b8ccf4c3c49987b92c8d

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Add if_packet.h headers

Some structs for implementing af_packet(7)

Some structs for implementing af_packet(7)