bpo-38353: Add subfunctions to getpath.c (GH-16572)

Following symbolic links is now limited to 40 attempts, just to
prevent loops.

Add subfunctions:

* Add resolve_symlinks()
* Add calculate_argv0_path_framework()
* Add calculate_which()
* Add calculate_program_macos()

Fix also _Py_wreadlink(): readlink() result type is Py_ssize_t, not
int.

Python/fileutils.c

@@ -1668,8 +1668,9 @@ _Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t buflen)
 {
     char *cpath;
     char cbuf[MAXPATHLEN];
+    size_t cbuf_len = Py_ARRAY_LENGTH(cbuf);
     wchar_t *wbuf;
-    int res;
+    Py_ssize_t res;
     size_t r1;
 
     cpath = _Py_EncodeLocaleRaw(path, NULL);
@@ -1677,11 +1678,12 @@ _Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t buflen)
         errno = EINVAL;
         return -1;
     }
-    res = (int)readlink(cpath, cbuf, Py_ARRAY_LENGTH(cbuf));
+    res = readlink(cpath, cbuf, cbuf_len);
     PyMem_RawFree(cpath);
-    if (res == -1)
+    if (res == -1) {
         return -1;
-    if (res == Py_ARRAY_LENGTH(cbuf)) {
+    }
+    if ((size_t)res == cbuf_len) {
         errno = EINVAL;
         return -1;
     }