minijail: Add a new option to allow program run as pid 1.

Add a new flag that indicates whether to fork or not
when pid namespace is set, so that programs can be
run as pid 1 inside a new pid namespace.

BUG=chromium:350616
TEST=security_Minijail0 pass
TEST=`minijail -I /bin/bash` then `echo $$` and get pid 1

Change-Id: Icc959b775e5fe6368c15a834e23ce3f2c119af41
Reviewed-on: https://chromium-review.googlesource.com/289440
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Yu-hsi Chiang <yuhsi@google.com>
Tested-by: Yu-hsi Chiang <yuhsi@google.com>
diff --git a/libminijail.h b/libminijail.h
index 6738a32..324731f 100644
--- a/libminijail.h
+++ b/libminijail.h
@@ -56,6 +56,7 @@
  */
 void minijail_namespace_pids(struct minijail *j);
 void minijail_remount_readonly(struct minijail *j);
+void minijail_run_as_init(struct minijail *j);
 void minijail_inherit_usergroups(struct minijail *j);
 void minijail_disable_ptrace(struct minijail *j);