www/caddy: Add HTTP keepalive to handler, revert NTLM deprecation.

[Monviech]

Some applications need a longer keepalive or the keepalive turned off. (Also known as connection timeout)

(e.g. Exchange Active Sync, other Microsoft services.)

keepalive https://caddyserver.com/docs/caddyfile/directives/reverse_proxy#keepalive

This only changes transport http. In transport http_ntlm the keepalive is turned off automatically, so this setting won't be rendered there.

NTLM is moved back to standard options since it can't be replaced.