…ddress feedback from MR.

* os.readlink() now returns a `str` (rather than `bytes`) when called with a file descriptor.
* os.readlink now added to `os.supports_fd` on the platforms where this feature is supported.
* Small refactor inside `os.readlink`: drop *_unavailable variables, generate error messages inline.
  Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS around each syscall site.
* Docs: Android is now mentioned as supported platform for this feature in all relevant places.

Emscripten apparrently defines O_PATH and readlinkat - even though it doesn't actually support them.
This broke auto-detection of whether os.readlink supports FDs - which broke the tests.

* Simplified/unified error handling in case reading fds referring to symlinks is not supported.
* Docs: centralized the information on the return type of os.readlink in a single paragraph.
* Docs/comments: rewrite from "MacOS" to "macOS" (lower-case).