MFC r339409, r339420:

MFC r339409, r339420:

  Add a new jail permission, allow.read_msgbuf.  When true, jailed processes
  can see the dmesg buffer (this is the current behavior).  When false (the
  new default), dmesg will be unavailable to jailed users, whether root or
  not.

  The security.bsd.unprivileged_read_msgbuf sysctl still works as before,
  controlling system-wide whether non-root users can see the buffer.

PR:		211580
Submitted by:	bz