Base64-encode %policy files to ensure it can be presented as strings

Base64-encode %policy files to ensure it can be presented as strings
- RPMTAG_POLICIES is a string array but there's no guarantee that
  something marked %policy is a plaintext file that can be represented
  as \0-terminated string, base64-encoding them fixes that. Baby steps
  towards making %policy remotely usable, related to RhBug:505066.
- Also remove unnecessary failure code setting, processMetadataFile()
  assumes failure already, and dont try to insert NULL strings in case
  b64encode() or pgpArmorWrap() fails
(cherry picked from commit edbc9ead961fcbeb1733b47405041b653b521bcb)