MDEV-4815 - allow multiple  mysql_server_init() / mysql_server_end() in the same process, for embedded library.

- Reset  static variables that are used to signal "init done"  for DBUG, in dbug_end()
- Set string server variables to NULL after memory for the value is freed - avoids double free()
- fix DBUG_ASSERTs that happened during reinitialization.