init: allow disabling selinux via a kernel command line

Create a new "androidboot.selinux" option, to control how userspace
handles SELinux. This kernel command line can have three options:

* disabled
* permissive
* enforcing

"disabled" completely disables userspace support for SELinux. No
policy is ever loaded, nor is the SELinux filesystem /sys/fs/selinux
ever mounted.

"permissive" loads the SELinux policy, but puts SELinux into
permissive mode. SELinux policy violations are logged, but not rejected.

"enforcing", the default, loads the SELinux policy, and places
SELinux into enforcing mode. Policy violations are rejected.

This change addresses post review comments for change
b710ed21 .

Change-Id: I912583db8e6a0e9c63380de32ad8ffc47a8a440f