FreeRDP version 2.0.0-rc0 and older will automatically free whatever
entry_points->pInterface is set to when the channel plugin is unloaded.
This doesn't happen in later versions, but will result in a double-free
upon disconnect for 2.0.0-rc0 in our case. As we don't need pInterface,
we can safely set this to NULL and avoid the issue entirely.
On some platforms, the libguacai-client.so plugin for FreeRDP reports an
unlinked symbol:
undefined symbol: guac_freerdp_dynamic_channel_collection_add (/usr/local/lib/freerdp2/libguacai-client.so)
This symbol is actually unused within the plugin, but may be referenced
due to being defined within a function in a common piece of source
shared between the plugin and the RDP support.
Separating the actual common components such that they can be included
by both the RDP support and the libguacai-client.so plugin removes the
potential for unused pieces being flagged as missing.
