add DUK_OPT_NO_FILE_IO and its DUK_USE_FILE_IO counterpart, make duk_push_string_file() sensitive to it

src/duk_api.c

@@ -2289,6 +2289,7 @@ const char *duk_push_string(duk_context *ctx, const char *str) {
 	}
 }
 
+#ifdef DUK_USE_FILE_IO
 /* This is a bit clunky because it is ANSI C portable.  Should perhaps
  * relocate to another file because this is potentially platform
  * dependent.
@@ -2333,6 +2334,14 @@ const char *duk_push_string_file(duk_context *ctx, const char *path) {
 	DUK_ERROR(thr, DUK_ERR_TYPE_ERROR, "failed to read file");
 	return NULL;
 }
+#else
+const char *duk_push_string_file(duk_context *ctx, const char *path) {
+	duk_hthread *thr = (duk_hthread *) ctx;
+	DUK_ASSERT(ctx != NULL);
+	DUK_ERROR(thr, DUK_ERR_TYPE_ERROR, "file I/O disabled");
+	return NULL;
+}
+#endif  /* DUK_USE_FILE_IO */
 
 void duk_push_pointer(duk_context *ctx, void *val) {
 	duk_tval tv;

src/duk_features.h

@@ -971,6 +971,17 @@ extern double duk_computed_nan;
 #define DUK_USE_DPRINT_COLORS
 #endif
 
+/*
+ *  File I/O support.  This is now used in a few API calls to e.g. push
+ *  a string from file contents or eval a file.  For portability it must
+ *  be possible to disable I/O altogether.
+ */
+
+#undef DUK_USE_FILE_IO
+#if !defined(DUK_OPT_NO_FILE_IO)
+#define DUK_USE_FILE_IO
+#endif
+
 /* 
  *  Profile processing
  *