Minor updates
- NuttX support courtesy of @xiaoxiang781216
- WolfSSL support courtesy of @xiongyu0523
- OpenVMS support courtesy of @Monarda
MQTTC_PAL_FILEpreprocessor definition can be used to specify a custom PAL file, courtesy of @vpetrigo- Minor tweaks/updates courtesy of @learn-more, @mpfj, @przemyslawzygmunt, @yamt, @underhood, @timgates42