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
5 files changed