labeled /proc/kmsg as proc_kmsg, changed logd's access from proc to
proc_kmsg, and added a compat mapping.

Bug: 65643247
Test: device boots without selinux denials to the newly introduced proc_kmsg
Test: logd-unit-tests passes
Change-Id: I92c9f5694289eb6a94c4d90f14e2de4d46b5228e