add a 'mangle prefix' option to allow people to tune the number of

add a 'mangle prefix' option to allow people to tune the number of
characters used in the prefix for 8.3 names in the hash2
algorithm. The longer the prefix the more readable the 8.3 names will
be, but the weaker the hash.

this was added because of someone complaining that the new hashing
algorithm was unreadable but the old one was broken :)
(This used to be commit 3ca3cc838e5b957c7244b21947daddc4ee4c3099)