[Samba] Why is S4 attempting to execute rndc when using *INTERNAL* DNS?

[Adam Tauno Williams]

My AD domain (SerNET S4 4.5.3) using S4's internal DNS, and always  
has.  So why is it now constantly logging an error of -

   [2017/01/13 12:53:00.152754,  0]  
../lib/util/util_runcmd.c:316(samba_runcmd_io_handler)
     /usr/sbin/rndc: Failed to exec child - No such file or directory

  - after attempting to add a 2008R2 DC.  I do not recall seeing this  
error previously [not prior to 4.5.3].