Revert "Add the S_{Begin/End}Registration syscalls and use them in cgame"

This reverts commit 888c89a24186627485458b25036f99ae4fd3d9be.
Merge branch 'master' into for-0.25
Revert "Revert "Add the S_{Begin/End}Registration syscalls and use them in cgame""

This reverts commit 6d07d5ca4099dee583543434c6403698843a041f.