Improve salt type display in kadmin getprinc

In krb5_salttype_to_string, output the salt type name we would
recognize as input.

In the output of getprinc, display the enctype and salt type in a form
we would accept--either enctype:salttype if the salt type is not the
default, or just the enctype if it is.

Update t_mkey.py and t_salt.py to expect the new output format.

ticket: 5958