Add HTTP transport for traffic

HTTP transport uses base64 encoding and POST requests to encapsulate
messages.  krb5.conf syntax is now:
`kdc = [(tcp|udp|http)://]<addr.of.kdc>[:<port>]`.
Enable kpasswd protocol to use HTTP for transport

Forces kpasswd to respect the transport protocol defined in krb5.conf
instead of ignoring it and choosing UDP.
Allow specification of page to POST to for HTTP

krb5.conf syntax is now:
`kdc = [(tcp|udp|http)://]<addr.of.kdc>[:<port>][/<page>]`
Add HTTPS transport for traffic

krb5.conf syntax is now:
`kdc = [(tcp|udp|http|https)://]<addr.of.kdc>[:<port>][/<page>]`
Update krb5.conf manpage for new kdc syntax