FileAnalysis: add custom libmagic database.

- It's derived from the magic database of libmagic 5.14, but with most
  everything not related to mime types removed.

- The custom database is always used by default for mime detection, but
  the more verbose file type detection will fall back on the default
  libmagic installation's database.  The result is: mime type strings
  are now guaranteed to be consistent across platforms, but the verbose
  file type descriptions are not.

- The custom database gets installed in $prefix/share/bro/magic, and
  should even be extensible if files with new patterns are added inside
  the directory.

- The search path for the mime magic database can be controlled via
  BROMAGIC environment variable.

- Remove mime_desc field from ftp.log.

- Stop using the mime/file type canonifier with unit tests.

- libmagic >= 5.04 is now a requirement.

testing/scripts/file-analysis-test.bro

@@ -49,10 +49,8 @@ event file_new(f: fa_file)
 
 	if ( f?$mime_type )
 		{
-		print "FILE_TYPE";
-		print "mime type is set";
-		# not actually printing the values due to libmagic variances
-		f$mime_type = "set";
+		print "MIME_TYPE";
+		print f$mime_type;
 		}
 	}