This commits adds a second FreeBSD 12 build job,
and splits the implementation of the FreeBSD module
into two modules, one for FreeBSD 11, and one for FreeBSD 12.

The FreeBSD 11 module is compiled always by default, and is
mostly forward compatible with FreeBSD 12 systems.

The FreeBSD 12 module is only built for now in libc's CI,
and uses FreeBSD 12 data types and APIs, linking to symbols
that are only available in FreeBSD 12.

Basically, when LIBC_CI env variable is defined, and the host
system is a FreeBSD 12 system, then the FreeBSD 12 module is
automatically built and tested. Conditional compilation is done
using a `cfg(freebsd12)` flag.

This commit also re-enables many tests, and documents why
some remain disabled.

Deprecate `use_std` cargo feature: use `std` instead .

Related to #657 .

Fix locale_t in unix and fuchsia.

Closes #1055.

Closes #1055.

Run CMSG tests on s390x

Closes #1240.

Closes #1240.

Related to #657 .

Fix Android build jobs

Non-existent constants like SYS_syscalls have been removed

* Update Android NDK to version r19c
* Update Android API versions to:
  * API 24 on arm and aarch64 targets
  * API 28 on x86 and x86_64 targets
* Unified headers were removed in NDK 16
* Refactor the NDK and SDK installation scripts
* OpenJDK version 8 must be kept: it appears that
  the Android tools do not work with more modern OpenJDK
  versions.

openbsd: add KERN_PFSTATUS and update KERN_MAXID

`KERN_PFSTATUS` has been recently added, and so `KERN_MAX` value has changed.
Add it to avoid failure on (local) ci.

While here, I added few blank lines to separated series of constants.

Netlink constants for use with the route netlink subsystem

Reopening a PR to close #1059. I'll be adding a few more constants but want to make sure CI works on Github first.

Refactor OpenBSD-like module into OpenBSD

The only OpenBSD-like OS that we support after bitrig's removal is OpenBSD itself.

cc @semarie