Add KCM_OP_GET_CRED_LIST for faster iteration

Add KCM_OP_GET_CRED_LIST for faster iteration

For large caches, one IPC operation per credential dominates the cost
of iteration.  Instead transfer the whole list of credentials to the
client in one IPC operation.

Add optional support for the new opcode to the test KCM server to
allow testing of the main and fallback code paths.

[ghudson@mit.edu: fixed memory leaks and potential memory errors;
adjusted code style and comments; rewrote commit message; added
kcmserver.py support and tests]

ticket: 8990 (new)