Allow config of dh_min_bits < 2048

Allow configuration to override the default dh_min_bits of 2048 to
1024.  Disallow configuration of dh_min_bits < 1024, but continue to
default to 2048.

https://github.com/krb5/krb5/commit/cae44d2d014985022a001924dce4a56d12c63818
Author: Tom Yu <tlyu@mit.edu>
Commit: cae44d2d014985022a001924dce4a56d12c63818
Branch: master
 src/plugins/preauth/pkinit/pkinit.h     |    1 +
 src/plugins/preauth/pkinit/pkinit_srv.c |    7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)