Be more tolerant of missing information

Instead of failing when IPv6 information is not found we
should be more tolerant of said information not existing
so that we behave like the pre IPv6 addition.
Fix the getgateway function

After the routeinfo function started to return
a dictionary containing ipv4 and ipv6 information
we now need to make sure we search the appropriate
key.
Fix the ipv6 header centering
Fix some whitespace issues
Cleanups for netinfo.py

- Handle ipv6 route information not existing
  gracefully (for systems that don't have it)
- Fix the getgateway function (broken due to
  ipv4/ipv6 keys now existing in route info)
- Separate the centering of the route info ipv4
  information from the centering of the ipv6 information
  so that this looks prettier...
- Use try: except: else instead of settings value to
  None and then later checking for None (more pythonic
  this way)