Move IAX firmware related functionality into separate files.

This patch is mostly a reorganization of existing code with a few exceptions:

* Added doxygen comments to all of the extracted functions.

* Split reload_firmware(int unload) into iax_firmware_reload() and
  iax_firmware_unload() for readability.

* Create iax_firmware_traverse() to support the 'iax2 show firmware' CLI
  command.

* Renamed iax_check_version() to iax_firmware_get_version() and change its
  arguments and return value so that it returns a success/failure value and sets
  the selected version into an out parameter to avoid confusion with failure and
  version 0.


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@380695 f38db490-d61c-443f-a65b-d21fe96a405b