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]

ticket: 8007 (new)
target_version: 1.13
tags: pullup