build,src: Add CloudABI as a POSIX-like runtime environment.

CloudABI is a compact POSIX-like runtime that makes use of
capability-based security. More details:
https://github.com/NuxiNL/cloudlibc

* src: Disable use of pwd.h, grp.h and get*uid() on CloudABI.
As CloudABI is intended to run applications in cluster contexts (e.g.,
on Kubernetes), they are oblivious of UNIX credentials. Extend the
existing preprocessor checks to disable any use of these interfaces,
just like on Windows, Android, etc.

* src: Explicitly include <netdb.h>.
cares_wrap.cc calls into functions like getnameinfo() and getaddrinfo().
These functions tend to be available implicitly through <uv.h>, but we'd
better still include this header explicitly.

On CloudABI, we make use of a custom implementation of libuv that does
not implicitly include header files like <netdb.h>.

PR-URL: https://github.com/nodejs/node/pull/16612


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>