emulator: Add option to control SELinux enforcement.

This change adds the following command line
options to the emulator:

  -selinux disabled
  -selinux permissive

This configures SELinux in either permissive or disabled modes.

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.