Make sysfs behave like netmount and localmount

sysfs now mounts all related sysfs file systems and returns success,
like netmount and localmount.

Also, we now check to make sure the cgroups are not mounted before we
mount them.

X-Gentoo-Bug: 530138
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=530138