steam-api: use PACKAGE macro in STEAM_API_AGENT
steam-http: use GHashTables over GTrees for headers and parameters

GHashTables should be quicker than GTrees as GTrees are based on binary
trees rather than hash tables to preserve ordering. The current usage
case is to store the headers and parameters, without retaining order.
steam-http: removed usage of steam_http_req_free_noreq
steam-http: minor refactoring
steam-api: compare API return values ignoring case
configure: added an argument for debugging features

Debugging features:
  - Enable all compiler warnings
  - Enable compiler debugging
  - Enable verbose HTTP replies
configure: ensure the compiler uses the C99 standard

The C99 standard is needed for the usage of designated initializers.

This is done by making use of the AC_PROG_CC_C99 macro which is present
in autoconf >= 2.59. On systems with older autoconf versions, this macro
will not be called.
steam-api: fixed the handling of "Not Logged On" errors

Currently, the implementation for checking for the error is broken due
to the the steam_api_relogon_check function. Additionally, the errors
were not being handled upon a poll reply.