kfw fix: restructure low ticket warning popup code to workaround mfc bug

kfw fix: restructure low ticket warning popup code to workaround mfc bug

mfc bug causes assertions when dialog is generated from
within PreTranslateMessages() (MSG input param points to a global
variable which is corrupted in the dialog message loop).  So we need
to instead PostMessage() to cause the popup later.
Also fixed logic to cause warning dialog to actually be modal as intended
when the leash window is not minimized.

Signed-off-by: Kevin Wasserman <kevin.wasserman@painless-security.com>
ticket: 7050

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25561 dc483132-0cff-0310-8789-dd5450dbe970