These constants have already been deprecated for a few releases with
a deprecation notice, so they can finally be removed.

Closes rust-lang/libc#665

PR #849 just moved all of the Linux structures into `src/fuchsia`.
While this is good for the most part, we really don't want the `SYS_*`
constants exported on Fushia, as these are not Fuchsia's syscalls.

This also removes Linux-specific `GRND_*` constants.

sigevent structs on most platforms have padding or unused fields. Rather
than display those in the Debug impl by deriving it, manually implement
all extra_traits instead ignoring those fields.

`nl_pad` field does not contain any actual data, so using it for
comparison or hashing doesn't make sense. Instead manually implement
extra traits ignoring this field.

The `pad` or `__reserved`  fields are not always 0 on some platforms,
so when used in the `PartialEq` implementation being used, fails some
comparisons. This commit manually implements the extra traits to
correct this behavior.

Closes #1055.

This PR fixes the build on all platforms and all Rust version down to the
minimum Rust version supported by libc: Rust 1.13.0.

The `build.rs` is extended with logic to detect the newer Rust features used by
`libc` since Rust 1.13.0:

* Rust 1.19.0: `untagged_unions`. APIs using untagged unions are gated on
  `cfg(libc_unions)` and not available on older Rust versions.

* Rust 1.25.0: `repr(align)`. Because `repr(align)` cannot be parsed by older
  Rust versions, all uses of `repr(align)` are split into `align.rs` and
  `no_align.rs` modules, which are gated on the `cfg(libc_align)` at the top
  level. These modules sometimes contain macros that are expanded at the top
  level to avoid privacy issues (`pub(crate)` is not available in older Rust
  versions). Closes #1242 .

* Rust : `const` `mem::size_of`. These uses are worked around with hardcoded
  constants on older Rust versions.

Also, `repr(packed)` structs cannot automatically `derive()` some traits like
`Debug`. These have been moved into `s_no_extra_traits!` and the lint of missing
`Debug` implementations on public items is silenced for these. We can manually
implement the `extra_traits` for these in a follow up PR. This is tracked
in #1243. Also, `extra_traits` does not enable `align` manually anymore.

Since `f64::to_bits` is not available in older Rust versions, its usage
has been replaced with a `transmute` to an `u64` which is what that method
does under the hood.

Closes #1232 .

This was not compile-tested on all platforms, but instead all `pub enum`
types had a `Debug` impl derived for them.

Linux 4.18 added support for SIGSYS info in signalfd. Add the new
fields to signalfd_siginfo.

While the kernel has support for these new fields now, no libc has
shipped a release with the new signalfd fields.

Remove false support for power and mips.
Fix aarch64 definitions of nlink_t and blksize_t.

Recent change #925 added constants for IPV6 multicast, but seems
to have inadvertently left off IPV6_UNICAST_HOPS from fuchsia.

Fixes #932