Issue #10914: Initialize correctly the filesystem codec when creating a new
subinterpreter to fix a bootstrap issue with codecs implemented in Python, as
the ISO-8859-15 codec.

Add fscodec_initialized attribute to the PyInterpreterState structure.
diff --git a/Python/pystate.c b/Python/pystate.c
index 586b856..b347c41 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -79,6 +79,7 @@
         interp->codec_search_cache = NULL;
         interp->codec_error_registry = NULL;
         interp->codecs_initialized = 0;
+        interp->fscodec_initialized = 0;
 #ifdef HAVE_DLOPEN
 #ifdef RTLD_NOW
         interp->dlopenflags = RTLD_NOW;