Flag commands r_portal_fastpath 0 and r_skybox_draw_last 0 as FCVAR_SERVER_CAN_EXECUTE

[vrad-exe]

The commands r_portal_fastpath and r_skybox_draw_last should be always allowed to be changed by servers in co-op, as these were previously disabled by a large number of community maps to fix certain rendering issues:

• With r_portal_fastpath enabled, the game's rendering will completely break down if more than 4 portals are on-screen, including linked_portal_doors. Lots of custom maps, including some made in BEEmod, require more than this for gameplay or technical reasons. Disabling the cvar removes this limit.
• With r_skybox_draw_last enabled, skyboxes will draw on top of world geometry, or if there is a 3D skybox it will draw on top of the 3D skybox geometry. This can alternatively be worked around by changing the shader used by the skybox, but this breaks HDR skyboxes and some other things. Disabling the cvar fixes this so skyboxes always render behind world geometry as expected.

Due to the recent change of FCVAR_SERVER_CAN_EXECUTE now being enforced, this no longer works in co-op, breaking many old and new maps which rely on them. Both of these cvars have very well known and tested effects, and reset when the game is closed, so there is little risk of changing these causing permanent breakage to a player's game - at worst, you would just need to restart the game and it would be reset.