properly randomize the message id

properly randomize the message id

We would only use the queue id of the message as its message id, which
is generated from the inode number of the message's queue file.  Inode
numbers are readily reused on many file systems, which leads to
repeating message ids.

Create real randomized message ids instead.

Submitted-by:  Peter Pentchev <roam@ringlet.net> (earlier version)