Refactor: fencing: parse full result from synchronous fencer replies

stonith_send_command() now parses the full result from synchronous fencer
replies, and maps that to a legacy return code, rather than parse the legacy
return code directly.

The full result is not used yet, and won't be until we can break backward API
compatibility, since the API functions that call stonith_send_command()
currently return a legacy code.