Improve error for kadmind -proponly without iprop

If kadmind is run with the -proponly flag, check that iprop_enable is
set and output a reasonable error message if it isn't.  Otherwise we
see the confusing message "Invalid argument while initializing
network".

[ghudson@mit.edu: rewrote commit message; edited error message]

https://github.com/krb5/krb5/commit/20c0296e536b791d051db523cd067398a1c6441d
Author: sashan <anedvedicky@gmail.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 20c0296e536b791d051db523cd067398a1c6441d
Branch: master
 src/kadmin/server/ovsec_kadmd.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)