Add checks and fix prototypes

This patch adds some checks in protocol.
It also fixes invalid function prototypes
Add SetSystemPointer message

This patch adds functions to handle SetSystemPointer messages, this allows
to remove the client-side pointer.
Merge pull request #5 from hardening/system_pointer

System pointer + prototype fixes
Merge branch 'session-manager' of github.com:bmiklautz/freerds
xrdp-ng: fix session manager build system issues