Support macOS 11 native credential cache

Support macOS 11 native credential cache

Add an API credential cache implementation using the CCAPI stubs in
the macOS Kerberos framework, tailored to access the native
collections used by macOS 10.6 and later (KCM before macOS 11, XCACHE
afterwards).  Make API: the default ccache name for macOS 10.6 and
later.

[ghudson@mit.edu: used shared CCAPI credential conversion functions;
changed ptcursor behavior to match current Unix collection semantics;
adjusted naming and code style]

ticket: 9052 (new)