In ksu, handle typeless default_ccache_name values

When a configured or compiled-in default ccache name doesn't contain a
cache type and ':' as a prefix, add one to the writeable value that we
construct when we go to look it up.  This lets the rest of the
application always assume that it'll be there.

[ghudson@mit.edu: minor style changes]

(cherry picked from commit bda574576a5a2d0613fecf12d820e0adcbedf95c)

ticket: 8007
version_fixed: 1.13
status: resolved
Document clock skew tolerance for ticket times

KDC and application server checks on ticket start and expiration times
are subject to clock skew tolerance.  Document this grace period.

[tlyu@mit.edu: edit commit message, adjust wording to conform to
existing style, document start time clock skew]

(cherry picked from commit e56f3d43a746c198b1fd1889dc1211b9feedbfc3)

ticket: 8008
version_fixed: 1.13
status: resolved