pthread: Allow configuration of priority and stacksize

The expected usage is:
  esp_pthread_set_cfg(cfg);
  pthread_create()

If the inherit flag is set, then all subsequent threads forked by this
thread will also inherit this configuration. This avoids having to
change/prefix this for each and every pthread_create() call.

docs/en/api-reference/system/index.rst

@@ -18,6 +18,7 @@ System API
    Sleep Modes <sleep_modes>
    Base MAC address <base_mac_address>
    Over The Air Updates (OTA) <ota>
+   ESP pthread <esp_pthread>
 
 
 Example code for this API section is provided in :example:`system` directory of ESP-IDF examples.