Add an environment variable to set a global default User-Agent value.

I think it would be nice to have a global default value for the User-Agent header commonly used by Agents that make HTTP requests.
It can be used by Agents other than those based on WebRequestConcern if it makes sense for them to set an alternate User-Agent value.

The environment variable name (DEFAULT_HTTP_USER_AGENT) may look verbose, but this is because names starting with HTTP_ should be avoided in the context of Web app.