Perform atomic ccache refreshes when possible

Perform atomic ccache refreshes when possible

Allow ccache types to implement atomic replacement via a new replace
method (replacing the unused "move" vtable slot).  Make krb5_cc_move()
use this method when possible, falling back to non-atomic replacement.
Implement atomic replacement for FILE, DIR, MEMORY, and KCM (using a
new opcode, falling back when it is not implemented).

Use krb5_cc_move() in get_in_tkt.c when an output ccache is specified,
in kinit for ticket validation and renewal, and in kvno --out-cache.

Add a test program to exercise concurrent krb5_get_credentials() and
cache refresh.

This commit does not implement atomic replacement for KEYRING or for
gss_store_creds().

ticket: 7707