Add mxs_strerror

Thread-safe version of strerror; thread local buffer used for storing
the message. The performance penalty of a thread local buffer is not
likely to be significant, since this is only called in an error
situation that anyway is likely to interrupt the normal processing.

server/core/log_manager.cc

@@ -24,6 +24,7 @@
 #include <errno.h>
 #include <syslog.h>
 #include <maxscale/atomic.h>
+#include <maxscale/platform.h>
 
 #include <maxscale/hashtable.h>
 #include <maxscale/spinlock.h>
@@ -3011,3 +3012,10 @@ int mxs_log_message(int priority,
 
     return err;
 }
+
+const char* mxs_strerror(int error)
+{
+    static thread_local char errbuf[MXS_STRERROR_BUFLEN];
+
+    return strerror_r(error, errbuf, sizeof(errbuf));
+}