Add final rtnetlink constants

This is a follow up to the PR #1351 which is for issue #1059 as it was closed before I realized there's one last set of netlink constants needed for use with rtnetlink. I'm marking this as WIP until I've verified everything's good with CI and that I've included all necessary constants. I'll remove WIP once it's passing CI and it's ready for review.

Change datatype for some IFF_ constants

These were originally added as c_short types, but all other IFF_ constants
are c_int. This commit changes them to match all the other constants for
consistency.

Closes #915

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

Related to #657 .

These were originally added as c_short types, but all other IFF_ constants
are c_int. This commit changes them to match all the other constants for
consistency.

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.