Create KDC and kadmind log files with mode 0640

Create KDC and kadmind log files with mode 0640

In krb5_klog_init(), use open() and fdopen() to open log files so that
we can specify a mode.  Specify a mode which doesn't include the
group-write, other-read, or other-write bits even if the process umask
allows them.

[ghudson@mit.edu: wrote commit message, de-indented post-open setup
code]

ticket: 8344 (new)