HTTP && HTTPS transport

These changes add support for HTTP and HTTPS transport of Kerberos traffic.  A reference implementation of a KDC HTTP proxy can be found [here](https://github.com/frozencemetery/krb-proxies) in the server-side directory.  The syntax for specifying a KDC location has been extended as described in the commits, and the man page for krb5.conf has been updated accordingly.  HTTPS currently uses OpenSSL for cryptography (with future plans for NSS support), and a flag to control which cryptography to use (currently OpenSSL or none) has been added to configure.

A post has also been made to the krbdev mailing list for review of these changes on 2013-08-08.