Enable kpasswd protocol to use HTTP for transport.

Currently, if the other end of the HTTP proxy is not running on (effectively)
the same machine as the KDC, the result will be "kpasswd: Incorrect net
address changing password".
Add ability to POST to a specific page for HTTP transport.

This necessitates a slight change for krb5.conf.  Syntax is now `kdc =
[(tcp|udp|http)://]<addr.of.kdc>[:<port>][/<page/location>]` with the
understanding that <page/location> will be ignored when HTTP is not specified.
Add HTTPS transport for Kerberos traffic.

krb5.conf syntax is now `kdc =
[(tcp|udp|http|https)]://<addr.of.kdc>[:<port>][/<page>]` with the
understanding that page will be ignored if the protocol is not HTTP or HTTPS.