[krbdev.mit.edu #6751] Allow Microsoft HMAC-MD5 checksum types to use non-RC4 keys

[Greg Hudson via RT]

This was reported by Douglas E. Engert <deengert at anl.gov>.  Pointers to 
the krbdev discussion thread:

http://mailman.mit.edu/pipermail/krbdev/2010-June/009145.html
http://mailman.mit.edu/pipermail/krbdev/2010-July/009147.html