external/toybox commit a583afc812cf7be74ebab72294c8df485908ff04 started
having dmesg use /dev/kmsg, which is unreadable to the unprivileged
shell user. Revoke syslog(2) to the shell user for consistency.

The kernel dmesg log is a source of kernel pointers, which can leak
kASLR information from the kernel. Restricting access to kernel
information will make attacks against Android more difficult. Having
said that, dmesg information is still available from "adb bugreport", so
this change doesn't completely shutdown kernel info leaks.

This change essentially reverts us to the state we were in between Nov 8
2011 and May 27 2014. During that almost 3 year period, the unprivileged
shell user was unable to access dmesg, and there was only one complaint
during that time.

References:
* https://android.googlesource.com/platform/system/core/+/f9557fb
* https://android.googlesource.com/platform/system/sepolicy/+/f821b5a

TODO: Further unify /dev/kmsg permissions with syslog_read permissions.

Test: policy compiles, no dmesg output
Change-Id: Icfff6f765055bdbbe85f302b781aed2568ef532f