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

[cfriedt]

Note: Coding Guidelines failures are false positives

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.

This lets us do fun things like what is shown below in the Zephyr shell:

uart:~$ posix env 
  set    get    unset
uart:~$ posix env get 
uart:~$ posix env set FOO=BAR
uart:~$ posix env get
FOO=BAR
uart:~$ posix env get FOO
BAR
uart:~$ posix env unset FOO
uart:~$ posix env set " 42 BCD" "No Problem!"
uart:~$ posix env get
uart:~$ posix env set "_BcD" "No Problem!"
uart:~$ posix env get
_BcD=No Problem!
uart:~$ posix env unset _BcD

App and library code can then query values from the environment, as a relatively simple and portable way of steering the behaviour of code at runtime.

Some notes:

• "poorly conditioned" variable names are dropped silently in the shell, which matches bash (or at least zsh)
• the Zephyr shell does not perform shell variable expansion at this time; in other words echo "$VALUE" will not work as one might expect

Possible future improvements:

• Heh... half joking, but adding shell variable expansion (or even an actual POSIX shell)
• this implementation is optimized for size (a speed optimization could maybe use hash tables)
• another future optimization for safety would be to have a malloc-free implementation
• a possibly nice feature for the shell would be to enable tab completions of names with env get <tab>
• another future improvement would be to use Kconfig / iterable sections to optionally enable some default environment variables to be populated (e.g. HOME, PWD, ...).
• another future improvement might be to add hooks for "dynamic" environment variables like $RANDOM.

Fixes #66861
Fixes #66862
Fixes #66863
Fixes #66864

[cfriedt]

[ycsin]

LGTM

[cfriedt]

LGTM

Thanks - I'll rebase to fix merge conflicts 😄

[cfriedt]

@ycsin - I also created a sample application (borrowed from your uname sample)

[cfriedt]

• mitigate possible memory leak by only assigning non-NULL return value to environ following realloc()
• change _m_home and other macros to e.g. M_HOME in test suite