fs_query: read content only from readable files

When configuring fs_query to print the content of files it used to try so for
all files it found resulting in errors on files that are not readable. Now,
fs_query will check first whether a file is readable and skip printing the
content of those that are not.

Ref #4032

repos/gems/src/app/fs_query/main.cc

@@ -86,6 +86,7 @@ struct Fs_query::Watched_file
 				if (_rwx.writeable)
 					xml.attribute("writeable", "yes");
 
+				if (_rwx.readable)
 					if (query.attribute_value("content", false))
 						_gen_content(xml, alloc, dir);
 			});