msg: add get_{pid,random}_nonce() helpers

msg: add get_{pid,random}_nonce() helpers

In cases where we normally use a pid for a nonce, fall back to a random
value when the pid == 1 (i.e., we're in a container).  For the cases where
we use a random value, use the helper.

Signed-off-by: Sage Weil <sage@redhat.com>