Various minor fixes/style changes
launch: Remove -h flag
launch: Add -t flag for specifying the TTY to run on
launch: Kill child when swc-launch dies
Move the (de)activation signal to the swc struct

This way, we will receive the initial activation event during
initialization, and the launcher doesn't have to set master on newly
opened DRM FDs.
launch: Drop swc_ prefix from non-public functions/structures
launch: Don't allow opening of devices when not active
drm: Use render nodes for rendering
cursor_plane: Don't attempt to set cursor when not active
launch: Add -n flag to start compositor without switching to its VT
Style
Update README
launch: Exit with status 2 on usage error