gh-152132: Enhance Py_RunMain() and fix bugs (#153461)

* Check for signals more often. Previously, a pending exception could
  be removed by PyErr_Clear().
* Only call _PyInterpreterState_SetNotRunningMain() if
  _PyInterpreterState_SetRunningMain() has been called.
* Add pymain_error() and pyrun_error(): write the error message to
  sys.stderr instead of the C stream stderr.
* Convert _PyPathConfig_UpdateGlobal() PyStatus error to an
  exception.
* Simplify pymain_run_startup(): avoid strerror() which is decoded
  from the wrong encoding. Instead display the exception, Py_fopen()
  raises an OSError with the path object (and can raise other exceptions).
* Add pymain_set_path0() function.
* Add _PySys_FormatV() function.

Author: vstinner

Include/internal/pycore_sysmodule.h

@@ -21,6 +21,12 @@ extern int _PySys_SetIntMaxStrDigits(int maxdigits);
 
 extern int _PySysRemoteDebug_SendExec(int pid, int tid, const char *debugger_script_path);
 
+extern void _PySys_FormatV(
+    PyObject *key,
+    FILE *fp,
+    const char *format,
+    va_list va);
+
 #ifdef __cplusplus
 }
 #endif

Lib/test/test_cmd_line.py

@@ -314,7 +314,7 @@ def test_undecodable_code(self):
             # decodable from ASCII) and run_command() failed on
             # PyUnicode_AsUTF8String(). This is the expected behaviour on
             # Linux.
-            pattern = b"Unable to decode the command from the command line:"
+            pattern = b"python: Unable to decode the command from the command line:"
         elif p.returncode == 0:
             # _Py_char2wchar() decoded b'\xff' as '\xff' even if the locale is
             # C and the locale encoding is ASCII. It occurs on FreeBSD, Solaris