kinit: get_switched_ccache

Provide a new internal function called get_switched() to encapsulate
the algorithm for selecting a credential cache when the selected
ccache type supports switching.  There is no change in behavior for
UNIX which always calls krb5_cc_new_unique().  However, on Windows
alternate behavior is provided when the ccache type is API or MSLSA.

For the API ccache the default ccache name is stored in the Windows
registry which is shared across all logon sessions belonging to a
user.  For users that are members of the Administrators group this
includes both the UAC restricted and elevated sessions sharing the
same desktop.  It is very disconcerting when the elevated session obtains
credentials for the same client principal as the restricted session
and then all apps in the restricted session lose access to their
credential cache.   For Windows, the API credential caches are named
after the principal that is stored within them.  It provides for a
better end user experience.

For the MSLSA ccache tickets belonging to multiple principals are
all stored within the MSLSA ccache.  As a result, all attempts to
switch ccache names default back to the one and only one name.

Change-Id: I7865cd044cff01ff38ab107ec0961e42788fa073