Restrict the ability to set usermodehelpers and proc security settings.

Limit the ability to write to the files that configure kernel
usermodehelpers and security-sensitive proc settings to the init domain.
Permissive domains can also continue to set these values.

The current list is not exhaustive, just an initial set.
Not all of these files will exist on all kernels/devices.
Controlling access to certain kernel usermodehelpers, e.g. cgroup
release_agent, will require kernel changes to support and cannot be
addressed here.

Expected output on e.g. flo after the change:
ls -Z /sys/kernel/uevent_helper /proc/sys/fs/suid_dumpable /proc/sys/kernel/core_pattern /proc/sys/kernel/dmesg_restrict /proc/sys/kernel/hotplug /proc/sys/kernel/kptr_restrict /proc/sys/kernel/poweroff_cmd /proc/sys/kernel/randomize_va_space /proc/sys/kernel/usermodehelper
-rw-r--r-- root     root              u:object_r:usermodehelper:s0 uevent_helper
-rw-r--r-- root     root              u:object_r:proc_security:s0 suid_dumpable
-rw-r--r-- root     root              u:object_r:usermodehelper:s0 core_pattern
-rw-r--r-- root     root              u:object_r:proc_security:s0 dmesg_restrict
-rw-r--r-- root     root              u:object_r:usermodehelper:s0 hotplug
-rw-r--r-- root     root              u:object_r:proc_security:s0 kptr_restrict
-rw-r--r-- root     root              u:object_r:usermodehelper:s0 poweroff_cmd
-rw-r--r-- root     root              u:object_r:proc_security:s0 randomize_va_space
-rw------- root     root              u:object_r:usermodehelper:s0 bset
-rw------- root     root              u:object_r:usermodehelper:s0 inheritable

Change-Id: I3f24b4bb90f0916ead863be6afd66d15ac5e8de0
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>