CRC -> MD5

[Hallvard B Furuseth <h.b.furuseth@usit.uio.no>]

We can drop the new CRC code in back-ldif in favor of the stronger MD5
from liblutil.  They have the same speed on my host: CRC is simpler,
but memory-bound.  CRC guarantees to catch certain transmission errors
like a few wrong bits, but this comes at the expense of its quality as
a general hash function.
    
Or we could use a slower SHA function, but I figure MD5 is already
stronger than we need.

-- 
Hallvard