Refactor AS-REQ nonce and timestamp handling

Refactor AS-REQ nonce and timestamp handling

Create helper functions to set the request nonce and to set the
request timestamp.  Don't bother picking a nonce in
restart_init_creds_loop since we will just pick a new one in
init_creds_step_request.  Create a library-internal function to get
the current time with possible adjustment from a preauth-required
error.  Only set ctx->request_time in one place (just before encoding
each request).  Remove unused parameters from stash_as_reply.

Partially based on a patch from Stef Walter.