improve message ID generation: put together a string based on user-data

improve message ID generation: put together a string based on user-data
(username, IP, remote port), serial data (time in ms) and randomness,
and put this all through a one-way hash. This makes the chance on
collisions unrealistically small, and at the same time prevents all
kinds of trouble when some vars are not available or contain non-allowed
information (like IPv6 addresses).

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12929 7612ce4b-ef26-0410-bec9-ea0150e637f0