problem: User-Id field in zap response is not populated

[JustinAzoff]

problem: zauth tests do not check User-Id metadata

Solution: update the PLAIN and CURVE tests to fetch and verify the
User-Id metadata set by ZAP

problem: User-Id field in zap response is not populated.

According to ZAP spec:

user id: this MAY provide the user identity in case of a 200 status, for
use by applications. For other statuses, it SHALL be empty.

Solution: This adds a char pointer in the zap request struct that can
point to the the username, client_key, or principal fields.

With this change zframe_meta(frame, "User-Id") returns the public key
for a connection. This enables a use case where you have a long lived
connection authenticated by a key or password and want to verify that the
user/key still exists.

zframe_t *frame = zframe_recv(server);
const char *user_id = zframe_meta(frame, "User-Id");
if(user_id!=NULL) {
    zcert_t *c = zcertstore_lookup(store, user_id);
    if(!c)
        //Certificate no longer exists in store!
}