Fixes for KFreeBSD and the Hurd:

- Issue #21274: Define PATH_MAX for GNU/Hurd in Python/pythonrun.c.

- Issue #21276: posixmodule: Don't define USE_XATTRS on KFreeBSD and the Hurd.

- Issue #21275: Fix a socket test on KFreeBSD.

Python/pythonrun.c

@@ -35,6 +35,10 @@
 #define PATH_MAX MAXPATHLEN
 #endif
 
+#ifdef __gnu_hurd__
+#define PATH_MAX MAXPATHLEN
+#endif
+
 _Py_IDENTIFIER(builtins);
 _Py_IDENTIFIER(excepthook);
 _Py_IDENTIFIER(flush);