steam-util: added g_prefix_error to keep compatible with glib < 2.16
steam-http: removed usage of g_uri_escape_string

This has been done for two reasons:
  1. The function is not in glib 2.14
  2. BitlBee already has its own internal version
Retrieve and store the session id upon authenticating

The session id, also known as a webcookie, is the authenticated value
needed to make calls to the Steam Community API. This will be used for
adding and removing friends.

This will require all accounts to reauthenticate.