Make the crc32 hash provider correctly chain multiple input buffers,

Make the crc32 hash provider correctly chain multiple input buffers,
so that it returns the same result if you pass it one big buffer or
many small buffers containing the same data.  To do this, change the
contract of mit_crc32 so that the cksum parameter is in-out.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23386 dc483132-0cff-0310-8789-dd5450dbe970