honor SELinux labels, when creating and writing config files

Also split out some fileio functions to fileio.c and provide a SELinux
aware pendant in fileio-label.c

see https://bugzilla.redhat.com/show_bug.cgi?id=881577

src/sysctl/sysctl.c

@@ -34,6 +34,7 @@
 #include "hashmap.h"
 #include "path-util.h"
 #include "conf-files.h"
+#include "fileio.h"
 
 static char **arg_prefixes = NULL;