Restore the ability to read legacy K*+157+* files

Restore the ability to read legacy K*+157+* files

The ability to read legacy HMAC-MD5 K* keyfile pairs using algorithm
number 157 was accidentally lost when the algorithm numbers were
consolidated into a single block, in commit
09f7e0607a34d90eae53f862954e98c31b5ae532.

The assumption was that these algorithm numbers were only known
internally, but they were also used in key files. But since HMAC-MD5
got renumbered from 157 to 160, legacy HMAC-MD5 key files no longer
work.

Move HMAC-MD5 back to 157 and GSSAPI back to 160.  Add exception for
GSSAPI to list_hmac_algorithms.