Use a "struct addrlist" instead of separate count and pointer-to-pointer values

Use a "struct addrlist" instead of separate count and pointer-to-pointer values
for internal interfaces for looking up servers.  Add a new routine to free the
addrlist contents, so that memory management can be changed in one place.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@13390 dc483132-0cff-0310-8789-dd5450dbe970