gh-98608: Change _Py_NewInterpreter() to _Py_NewInterpreterFromConfig() (gh-98609)

(see https://github.com/python/cpython/issues/98608)

This change does the following:

1. change the argument to a new `_PyInterpreterConfig` struct
2. rename the function to `_Py_NewInterpreterFromConfig()`, inspired by `Py_InitializeFromConfig()` (takes a `_PyInterpreterConfig`  instead of `isolated_subinterpreter`)
3. split up the boolean `isolated_subinterpreter` into the corresponding multiple granular settings
   * allow_fork
   * allow_subprocess
   * allow_threads
4. add `PyInterpreterState.feature_flags` to store those settings
5. add a function for checking if a feature is enabled on an opaque `PyInterpreterState *`
6. drop `PyConfig._isolated_interpreter`

The existing default (see `Py_NewInterpeter()` and `Py_Initialize*()`) allows fork, subprocess, and threads and the optional "isolated" interpreter (see the `_xxsubinterpreters` module) disables all three.  None of that changes here; the defaults are preserved.

Note that the given `_PyInterpreterConfig` will not be used outside `_Py_NewInterpreterFromConfig()`, nor preserved.  This contrasts with how `PyConfig` is currently preserved, used, and even modified outside `Py_InitializeFromConfig()`.  I'd rather just avoid that mess from the start for `_PyInterpreterConfig`.  We can preserve it later if we find an actual need.

This change allows us to follow up with a number of improvements (e.g. stop disallowing subprocess and support disallowing exec instead).

(Note that this PR adds "private" symbols.  We'll probably make them public, and add docs, in a separate change.)

Include/cpython/initconfig.h

@@ -213,10 +213,6 @@ typedef struct PyConfig {
     // If equal to 0, stop Python initialization before the "main" phase.
     int _init_main;
 
-    // If non-zero, disallow threads, subprocesses, and fork.
-    // Default: 0.
-    int _isolated_interpreter;
-
     // If non-zero, we believe we're running from a source tree.
     int _is_python_build;
 } PyConfig;
@@ -245,6 +241,21 @@ PyAPI_FUNC(PyStatus) PyConfig_SetWideStringList(PyConfig *config,
     Py_ssize_t length, wchar_t **items);
 
 
+/* --- PyInterpreterConfig ------------------------------------ */
+
+typedef struct {
+    int allow_fork;
+    int allow_subprocess;
+    int allow_threads;
+} _PyInterpreterConfig;
+
+#define _PyInterpreterConfig_LEGACY_INIT \
+    { \
+        .allow_fork = 1, \
+        .allow_subprocess = 1, \
+        .allow_threads = 1, \
+    }
+
 /* --- Helper functions --------------------------------------- */
 
 /* Get the original command line arguments, before Python modified them.

Include/cpython/pylifecycle.h

@@ -62,4 +62,5 @@ PyAPI_FUNC(int) _Py_CoerceLegacyLocale(int warn);
 PyAPI_FUNC(int) _Py_LegacyLocaleDetected(int warn);
 PyAPI_FUNC(char *) _Py_SetLocaleFromEnv(int category);
 
-PyAPI_FUNC(PyThreadState *) _Py_NewInterpreter(int isolated_subinterpreter);
+PyAPI_FUNC(PyThreadState *) _Py_NewInterpreterFromConfig(
+    const _PyInterpreterConfig *);

Include/cpython/pystate.h

@@ -3,11 +3,38 @@
 #endif
 
 
+/*
+Runtime Feature Flags
+
+Each flag indicate whether or not a specific runtime feature
+is available in a given context.  For example, forking the process
+might not be allowed in the current interpreter (i.e. os.fork() would fail).
+*/
+
+// We leave the first 10 for less-specific features.
+
+/* Set if threads are allowed. */
+#define Py_RTFLAGS_THREADS      (1UL << 10)
+
+/* Set if os.fork() is allowed. */
+#define Py_RTFLAGS_FORK         (1UL << 15)
+
+/* Set if subprocesses are allowed. */
+#define Py_RTFLAGS_SUBPROCESS   (1UL << 16)
+
+
+PyAPI_FUNC(int) _PyInterpreterState_HasFeature(PyInterpreterState *interp,
+                                               unsigned long feature);
+
+
+/* private interpreter helpers */
+
 PyAPI_FUNC(int) _PyInterpreterState_RequiresIDRef(PyInterpreterState *);
 PyAPI_FUNC(void) _PyInterpreterState_RequireIDRef(PyInterpreterState *, int);
 
 PyAPI_FUNC(PyObject *) _PyInterpreterState_GetMainModule(PyInterpreterState *);
 
+
 /* State unique per thread */
 
 /* Py_tracefunc return -1 when raising an exception, or 0 for success. */

Include/internal/pycore_interp.h

@@ -143,6 +143,7 @@ struct _is {
 #ifdef HAVE_DLOPEN
     int dlopenflags;
 #endif
+    unsigned long feature_flags;
 
     PyObject *dict;  /* Stores per-interpreter state */