Implement CSQC_SIMPLE aka hud-only CSQC, improve compatibility

Implement CSQC_SIMPLE aka hud-only CSQC, improve compatibility

This should match the behaviours of engines with minimal CSQC support,
allowing DP to run any csprogs made for them.

Fixes https://github.com/DarkPlacesEngine/darkplaces/issues/108

Updates dpdefs to include compatible parameter names.

Adds VM-specific code for checking required functions during QC program
loading.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>