r4032: Improvements for file type guessing (Thomas Leonard):

r4032: Improvements for file type guessing (Thomas Leonard):
- When getting the type for a symlink, we need the absolute path now that we
  check file contents.
- Also check for extended attributes for the symlink target.
- Removed all of our code for guessing types from file names, since it
  duplicates the new xdgmime code.
- Files with control characters are now considered to be binary, even
  though they may be valid UTF-8.
- Executable files of unknown type are now marked as executable, not
  octet-stream.
- When complaining that there is no run action for an executable type,
  give the user a hint about setting the execute permission.
- Refreshing the MIME system also refreshes xdgmime.
- Added MIME rules to say that Python, PERL and Ruby are executable, as
  old versions of the MIME database don't have these rules (reported by
  Stephen Watson).