Add KCM credential cache type (client only)

Add a new credential cache type "KCM" which performs cache operations
by speaking to a Heimdal or OS X KCM daemon, via either Unix domain
sockets or (on OS X only) Mach RPC.  Add "kcm_socket" and
"kcm_mach_service" profile variables to control the socket path and
bootstrap service name respectively.  In ccmarshal.c, add
k5_marshal_mcred to marshal matching credentials in the KCM protocol
representation.

This cache type is not currently supported on Windows, as Windows does
not support Unix domain sockets.

As with the keyring cache type, the lastchange method of this cache
type is mostly useless, reporting only the time of the last change
made through that cache handle.  The KCM protocol currently has no
support for obtaining the last change time of the cache itself.

ticket: 7964 (new)
Make KCM the default ccache on OS X

If we do not find a default ccache value from krb5-config and we
detect that the host platform is OS X 10.7 or higher, use KCM: as the
default ccache name instead of FILE:/tmp/krb5cc_%{uid}.

ticket: 7964
Document KCM ccache type

Document the new KCM ccache type in ccache_def.rst.  Document the
kcm_socket and kcm_mach_service variables in krb5_conf.rst.

ticket: 7964