posix: env: support for environ, getenv(), setenv(), unsetenv()

Support getting and setting POSIX environment variables.

Additionally, the thread-safe BSD variant getenv_r() is
provided.

environ, getenv(), setenv(), and unsetenv() are required by
the POSIX_SINGLE_PROCESS Option Group as detailed in
Section E.1 of IEEE-1003.1-2017.

The POSIX_SINGLE_PROCESS Option Group is required for PSE51,
PSE52, PSE53, and PSE54 conformance, and is otherwise mandatory
for any POSIX conforming system as per Section A.2.1.3 of
IEEE-1003-1.2017.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>