lib: posix: standard include paths with CONFIG_POSIX_API

With the `<zephyr/posix/...> prefix, it became
exponentially more difficult to integrate 3rd-party
libraries that depend on the POSIX API.

Standard POSIX headers should be available in standard
include paths - and that should most certainly the case
when `CONFIG_POSIX_API=y`.

With this change:

* When `CONFIG_POSIX_API=y`
  - applications have explicitly chosen to use
    POSIX APIs.
  - all standard POSIX includes are in the default
    include path.
* When `CONFIG_POSIX_API=n`
  - applications *may* include POSIX headers
    explicitly with the namespaced prefix
  - e.g. `#include <zephyr/posix/unistd.h>`
  - individual Kconfig options can be used to
    enable POSIX features selectively, such as
    `getopt` or `eventfd`.

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