Between `ClientRequest` and `Agent`. The circular require was doing
weird things at load time, like making the `globalAgent` property
be `undefined` from within the context of the "_http_client"
module.

Removing the circular dependency completely fixes this.

This commit effectively removes the undocumented `Agent#request()`
and `Agent#get()` functions.

Otherwise the string triggers an assertion error in node_http_parser.c,
line 370:

    assert(Buffer::HasInstance(args[0]) == true);

because the first argument is not a Buffer object.

If the socket was destroyed, we need to remove it from the agent's
`freeSockets` list, otherwise dead socket could be reused by new
request.

The `options` that were being passed in before here are specific to a
single request, which kinda defeats the purpose of using an Agent in the
first place.

On a worse note, these `options` have not yet been "processed" by the
`http.ClientRequest` class, so if `port: null` is set (like it is as the
result of a `url.parse()` call), then they take preference over the
processed values since the agent's "options" get mixed in last in the
`createSocket()` function.

Fixes #6197.
Fixes #6199.
Closes #6231.

There are some agent subclasses using this today.

Despite the addRequest function being undocumented internal API, it's
easy enough to just support the old signature for backwards
compatibility.

Noticed by @bnoordhuis in https://github.com/joyent/node/pull/5991#discussion_r5575946

Increases the grep factor. Makes it easier to harmonize type checks
across the code base.