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.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

Signed-off-by: Gerd Zellweger <gzellweger@vmware.com>

Signed-off-by: Gerd Zellweger <mail@gerdzellweger.com>

This used to be defined multiple times in different (but not all)
architectures. Moving it to general linux module since it's an
architecture independent feature.

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.

These are necessary for large file support on 32-bit platforms,
following the same pattern as 74825222 ("Add bindings for
-D_FILE_OFFSET_BITS=64").

Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>

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

Some structs for implementing af_packet(7)

The `ifaddrs` interface is available since api version 24 in android.
The function signatures are now equal to the standard Linux function
signatures.