minijail: Add support for entering an existing VFS namespace.

Also, fix the Makefile while in there.

BUG=chromium:376987
TEST=security_Minijail0
CQ-DEPEND=CL:209242

Change-Id: I18877211549500cbb720805a2480b1cb3244c1e9
Reviewed-on: https://chromium-review.googlesource.com/209240
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/libminijail.h b/libminijail.h
index 865a4f1..6738a32 100644
--- a/libminijail.h
+++ b/libminijail.h
@@ -49,6 +49,7 @@
 void minijail_log_seccomp_filter_failures(struct minijail *j);
 void minijail_use_caps(struct minijail *j, uint64_t capmask);
 void minijail_namespace_vfs(struct minijail *j);
+void minijail_namespace_enter_vfs(struct minijail *j, const char *ns_path);
 void minijail_namespace_net(struct minijail *j);
 /* Implies namespace_vfs and remount_readonly.
  * WARNING: this is NOT THREAD SAFE. See the block comment in </libminijail.c>.