[crashlog] Turn crash log parsing modes into a Python 'enum' (NFC)

Python doesn't support enums before PEP 435, but using a class with
constants is how it's commonly emulated. It can be converted into a real
Enum (in Python 3.4 and later) by extending the Enum class:

  class CrashLogParseMode(Enum):
      NORMAL = 0
      THREAD = 1
      IMAGES = 2
      THREGS = 3
      SYSTEM = 4
      INSTRS = 5