Move the handling of sv_cheats to common.cpp
Fix the client resetting sv_cheats too eagerly when starting a local server
Fix the VM not having the right value for g_cheats.

This used to work because the vmCvar_t could be registered at the same
cvar than an engine cvar. This is no longer the case so we just send the
value of sv_cheats when the map starts (and directly populate the
vmCvar_t).