Validate if a file is readable text when report_changes is set

Syscheckd will save in .<ossec>/queue/diff/ any file with report_changes
option, e.g. /chroot/dev/urandom, iso, mp3. This patch integrates libmagic
to validate mime type. Only mime type beginning with 'text', e.g. text/html,
text/plain, will be saved and reported by the report_changes option.