Add system call set_relative_mode

[alanjian85]

Add a new system call to allow users to control the mouse relative mode of the SDL window and to synchronize the cursor with the game demo menu's current state. This commit also updates the ELF executable files of Quake and Doom demo.

P.S. I also opened change requests against doom_riscv and quake-embedded

[jserv]

Can we stick to the principles of minimalist regarding the design of system? That is, instead of providing new system call, how about extending the semantics of poll_event by checking a certain bit which indicates the switch of the mouse relative mode in SDL.