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 (yes it really happened to me), iso, mp3. This patch integrates libmagic
to validate mime type. Only mime type beginning with 'text/', e.g. text/html,
text/plain, will be copied and reported by diff. 

This should pave the way for binary diff. ;)

Reviewers: I'm not quite sure about the build process (e.g. MEXTRA, MAGICCMD) so please advice.