Fix minor race in kadmin time interval parsing

Fix minor race in kadmin time interval parsing

When parsing kadmin time intervals using getdate.y relative time
formats, make sure the same timestamp is added to and substracted from
the relative value.  To accomplish this, rename get_date() to
get_date_rel() with a second parameter for the current time, and make
get_date() a wrapper with the current signature for the benefit of
kdb5_util and kdb5_ldap_util.

ticket: 8393