Fix mxb_strerror

The function should not be an inline function with a static variable. This
appears to cause problems on at least Debian Wheezy and is likely to cause
odd behavior on other platforms.

Also renamed the file to <maxbase/string.h> to better mirror how the
<string.h> file behaves.

maxutils/maxbase/src/string.cc

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxbase/string.h>
+
+#include <string.h>
+
+namespace
+{
+
+thread_local char errbuf[512]; // Enough for all errors
+
+}
+
+const char* mxb_strerror(int error)
+{
+#ifdef HAVE_GLIBC
+    return strerror_r(error, errbuf, sizeof(errbuf));
+#else
+    strerror_r(error, errbuf, sizeof(errbuf));
+    return errbuf;
+#endif
+}