steam-util: added g_strcmp0 to keep compatible with glib < 2.16

As of bitlbee 3.2, the required version of glib is only 2.14. This must
be the version that the plugin depends on.

A dependency was made on glib >= 2.16 by the usage of g_strcmp0. To keep
compatible, define g_strcmp0 if it is missing.
steam-http: compare headers and parameters while ignoring their casing

This prevents doubly storing headers and parameters that have the same
name, but different casing.
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.