Short-circuit MIT setup if krb5kdc not installed
On a system with some MIT utils installed but no KDC, the script went all the way down the MIT branch and only failed at kinit, by which time KDCPROC had been initialized to something invalid.
Let's short-circuit before it gets that far.
@iboukris please sanity check... is noting the Heimdal version valuable or was that there just for testing whether kdc
is executable?