posix: getopt: cannot use getopt() in a standard way

[cfriedt]

Describe the bug
Getopt should be defined in <unistd.h> according to the POSIX specification. Of course, in Zephyr, that header is available in <zephyr/posix/unistd.h>.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? all (aside from native_posix types)
• What have you tried to diagnose or workaround this issue? Many, many things.
• Is this a regression? If yes, have you been able to "git bisect" it to a
  specific commit? no, it's always been broken
• ...

To Reproduce
Steps to reproduce the behavior:

1. Compile the example program below
2. see failure

#include <zephyr/posix/unistd.h>

void foo(void)
{
  int argc = 42;
  char *const argv[] = {"So", "long", "and", "thanks", "for", "all", "the", "fish" };
  getopt(argc, argv, "");
}

Expected behavior

Impact
It's a showstopper for anyone expecting to be able to use standard POSIX code portably

Logs and console output
The warning below is promoted to error when run in twister

west build -p auto -b qemu_cortex_a53 -t run tests/posix/getopt
...
/Users/cfriedt/workspace/zephyrproject/zephyr/tests/posix/getopt/src/foo.c: In function 'foo':
/Users/cfriedt/workspace/zephyrproject/zephyr/tests/posix/getopt/src/foo.c:7:9: warning: implicit declaration of function 'getopt' [-Wimplicit-function-declaration]
    7 |         getopt(argc, argv, "");
      |         ^~~~~~

twister -i -p qemu_cortex_a53 -T tests/posix/getopt
...
/Users/cfriedt/workspace/zephyrproject/zephyr/tests/posix/getopt/src/foo.c: In function 'foo':
/Users/cfriedt/workspace/zephyrproject/zephyr/tests/posix/getopt/src/foo.c:7:9: error: implicit declaration of function 'getopt'; did you mean 'getsubopt'? [-Werror=implicit-function-declaration]
    7 |         getopt(argc, argv, "");
      |         ^~~~~~
      |         getsubopt

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows): any
• Toolchain (e.g Zephyr SDK, ...): Zephyr SDK 0.15.2
• Commit SHA or Version used: dc26a61

Additional context