Add userspace_panic_device and userpanic_use

Define type userspace_panic_device and macro userpanic_use for init,
llkd, and system_server to access /dev/userspace_panic - a kernel file
node for userspace processes to request kernel panic.

Bug: 188777408
Change-Id: I1e9d115d85f664aa84bdd6bb4b95bdb48e3aab9a
diff --git a/private/file_contexts b/private/file_contexts
index 0c8bf78..8849602 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -191,6 +191,7 @@
 /dev/uio[0-9]*		u:object_r:uio_device:s0
 /dev/urandom		u:object_r:random_device:s0
 /dev/usb_accessory	u:object_r:usbaccessory_device:s0
+/dev/userspace_panic	u:object_r:userspace_panic_device:s0
 /dev/v4l-touch[0-9]*	u:object_r:input_device:s0
 /dev/vhost-vsock	u:object_r:kvm_device:s0
 /dev/video[0-9]*	u:object_r:video_device:s0