steam-api: error upon empty summary results

Often times, summary requests are needed to retrieve additional friend
information. However, the Steam API occasionally returns an empty list
of summaries. This leads to a slew of problems for the plugin.

This patch fixes several issues:
  - Endless amount of summary requests
  - Huge amounts of HTTP request spam
  - The halting of certain API requests

The solution is to simply error when this issue occurs, and reconnect,
which typically resolves the issue.
Improved debugging implementations

This patch implements several changes that improve the debugging output,
as well as conform to bitlbee's debugging techniques.

Debugging improvements:
  - Added the SteamHttpReq memory address to the debugging output
  - Always dump HTTP responses, even on internal errors
  - Debugging now uses envvars BITLBEE_DEBUG or BITLBEE_DEBUG_STEAM
  - Renamed the DEBUG macro to DEBUG_STEAM
  - Updated the README with debugging information
steam-http: implemented request monitoring

Some HTTP requests fail to receive a response from the server. This
issue has become known with the keep-alive based requests, which are
currently used for polling operations. With poll requests, a response
is essential to the continuation of the polling loop, otherwise the
loop becomes broken.

This patch simply ensures that specified requests receive a response
within an alloted timeout. If a response is not received, an error is
raised on the request.
Fixed relogon and the underlying request resending

Currently, requests that are queued for resending when the queue is
paused are immediately sent as well as requeued. This is problematic,
as they are resent without the STEAM_HTTP_REQ_FLAG_NOFREE flag, and
this inevitably leads to segmentation faults. Moreover, the whole
resending logic needed a great deal of refactoring.

Additionally, much refactoring was done on the steam-api interface to
conform with similar hidden operations. This keeps the steam-api
interface consistent.

This fixes the client crashing upon the relogon operation.