Prevent hidden weapons being revealed with waypoints and centreprints

Prevent hidden weapons being revealed with waypoints and centreprints

Normally this issue isn't noticeable, but with
g_balance_blaster_weaponstart 0 the secret tubas were revealed by the
weapon_group_1 bind.

If all weapons in a group are HIDDEN, nothing is displayed when the
group bind is pressed.  This seems better than the alternatives:
- "AOL CD Thrower is not available"
- "@!#%'n Tuba is not available" even though it's in the secret room
- "No weapon in the group is available" but it's a lie