Allow Microsoft HMAC-MD5 checksum types to use non-RC4 keys

Allow Microsoft HMAC-MD5 checksum types to use non-RC4 keys

In PAC signatures, the hmac-md5 checksum type can be used with AES
keys.  Make this work by removing the enc field from the hmac-md5 and
md5-hmac checksum types, and adding a check in
krb5int_hmacmd5_checksum() for a null key or a key which is longer
than the hash block size (64 bytes for MD5).  The checksum algorithm
only uses the key bits; it does invoke the cipher.

The checksum type names are kind of wrong, but we'll leave them alone
for compatibility.  The descriptions are updated.

ticket: 6751
target_version: 1.8.3
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24191 dc483132-0cff-0310-8789-dd5450dbe970