Postmortem - export user reset reason & extra callbacks

ref. esp8266#9193, esp8266#9194, esp8266#9196

- persistent & public REASON_USER_... for user callback rst_info reason
- public postmortem callback functions, don't overly hide the implementation
- separate reasons for panic, assert, abort and c++ exceptions
- put postmortem func one level down in the function chain, allow to
  override both the function itself or individual internals
- stack offset for -fexceptions, skip unwind data to the actual point of origin

Author: mcspr