Provide the C WASI implementation as an option.

This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.

wasmtime-wasi-c/sandboxed-system-primitives/src/str.c

@@ -0,0 +1,33 @@
+// Part of the Wasmtime Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://github.com/CraneStation/wasmtime/blob/master/LICENSE for license information.
+//
+// Significant parts of this file are derived from cloudabi-utils. See
+// https://github.com/CraneStation/wasmtime/blob/master/lib/wasi/sandboxed-system-primitives/src/LICENSE
+// for license information.
+//
+// The upstream file contains the following copyright notice:
+//
+// Copyright (c) 2016 Nuxi, https://nuxi.nl/
+
+#include "config.h"
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "str.h"
+
+char *str_nullterminate(const char *s, size_t len) {
+  // Copy string.
+  char *ret = strndup(s, len);
+  if (ret == NULL)
+    return NULL;
+
+  // Ensure that it contains no null bytes within.
+  if (strlen(ret) != len) {
+    free(ret);
+    errno = EILSEQ;
+    return NULL;
+  }
+  return ret;
+}