Omit start time in common AS requests

Omit start time in common AS requests

MIT and Heimdal KDCs ignore the start time for non-postdated ticket
requests, but AD yields an error if the start time is in the KDC's
future, defeating the kdc_timesync option.  Omit the start time if the
caller did not specify a start time offset.

This change reenables the client check for too much clock skew in the
KDC reply in the non-timesync configuration.  That check had been
unintentionally suppressed since the introduction of the
get_init_creds interfaces.  Adjust the t_skew test script to expect
the new error behavior.

Code changes from stefw@gnome.org with slight modifications.

ticket: 7130

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25864 dc483132-0cff-0310-8789-dd5450dbe970