make it possible to use --enable-fs more than once

Change add_fsname_auto() to join multiple --enable-fs options.

Note: "all" always wins, and "--enable-fs foo,all,bar" results
in fsauto_names = "all" too.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>