CSM restrictions: Make 'LOAD_CLIENT_MODS' disable loading of 'builtin' (#8000)

CSM restrictions: Make 'LOAD_CLIENT_MODS' disable loading of 'builtin' (#8000)

Previously, when the CSM restriction 'LOAD_CLIENT_MODS' was used a
client was still able to add CSM code to 'builtin' to bypass that
restriction, because 'builtin' is not yet verified.

Until server-sent CSM and verifying of 'builtin' are complete, make
'LOAD_CLIENT_MODS' disable the loading of builtin.

Clarify code comments and messages to distinguish between client-side
modding and client-side scripting. 'Scripting' includes 'builtin',
'modding' does not.