Make ksu respect the default_ccache_name setting

Make ksu respect the default_ccache_name setting

Move the logic for resolving and initializing a cache that we're
copying creds into out of krb5_ccache_copy(), and let the caller deal
with it.  Add a helper functions to select/resolve an output ccache in
the default location for the target user after we've switched to the
target user's privileges.  If the destination is a collection, take
care not to change which subsidiary is its primary, and reuse a
subsidiary cache if we can.  If the destination is not a collection,
append a unique value to its name to make a new ccache.

[ghudson@mit.edu: some changes to variable names and comments; move
responsibility for getting target ccache name from
resolve_target_ccache to main]

ticket: 7984 (new)