build menu: really keep lights on bsp stage, rename -keepLights to -keeplights

build menu: really keep lights on bsp stage, rename -keepLights to -keeplights

The option was misnamed when introduced in 4491fb4 7 years ago:

https://gitlab.com/xonotic/netradiant-xonoticpack/-/commit/4491fb4e31cc854b2e9db0e68b155728ea11d9b9

Fortunately this option is enabled by default in q3map2 since more than 10 years commit e347299 at NetRadiant:

https://gitlab.com/xonotic/netradiant/-/commit/e34729907cc073c56d438c0f52d1c77cae9bcf94

Unfortunateky it means all maps built with this build menu since 7 years
did not keep lights…

Q3map2 printed this:

> WARNING: Unknown option "-keepLights"