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.

Change-Id: I97974deb5b39d5caab36032e8b282281c1e478ea
diff --git a/android/help.c b/android/help.c
index 0a9eed8..e5d3e49 100644
--- a/android/help.c
+++ b/android/help.c
@@ -1526,6 +1526,18 @@
 }
 
 static void
+help_selinux(stralloc_t* out)
+{
+    PRINTF(
+    "  Use -selinux to control the SELinux enforcement mode.\n"
+    "  By default, SELinux is in enforcing mode. Other modes available are:\n"
+    "     -selinux permissive   -> Load the SELinux policy, but do not enforce it.\n"
+    "                              Policy violations are logged, but not rejected.\n"
+    "     -selinux disabled     -> Disable kernel support for SELinux.\n"
+    );
+}
+
+static void
 help_force_32bit(stralloc_t* out)
 {
     PRINTF(