Improve acceptor name flexibility

Improve acceptor name flexibility

Be more flexible about the principal names we will accept for a given
GSS acceptor name.  Also add support for a new libdefaults profile
variable ignore_acceptor_hostname, which causes the hostnames of
host-based service principals to be ignored when passed by server
applications as acceptor names.

Note that we still always invoke krb5_sname_to_principal() when
importing a gss-krb5 mechanism name, even though we won't always use
the result.  This is an unfortunate waste of getaddrinfo/getnameinfo
queries in some situations, but the code surgery necessary to defer
it appears too risky at this time.

The project proposal for this change is at:

http://k5wiki.kerberos.org/wiki/Projects/Acceptor_Names

ticket: 6855

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24616 dc483132-0cff-0310-8789-dd5450dbe970