Prohibit execute to fs_type other than rootfs for most domains.

Augment the already existing neverallow on loading executable content
from file types other than /system with one on loading executable content
from filesystem types other than the rootfs.  Include exceptions for
appdomain and recovery as required by current policy.

Change-Id: I73d70ab04719a67f71e48ac795025f2ccd5da385
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>