(maint) Rename cfacter in sources to facter.

- Change libcfacter to libfacter; keeping cfacter as the executable for
  now so that there is no confusion with existing facter.
- Change "cfacter" namespace to "facter" in libfacter.
- Move headers into a "facter" subdirectory so that they can be included
  with a library name prefix like other libraries.
- Update include guards so that cpplint is happy.
- No functionality change other than logging namespace is now
  "puppetlabs.facter.XXX".