Improve R_AddNormals

New code results in a more accurate image because 128 / 255 and 127 / 255 do not exactly equal 0.5. There are also fewer and less expensive calculations so it should be faster.
The map thunder spent a significant amount of time in R_AddNormals when loading.
Remove useless rgb max calculations in R_UploadImage
renderer: ignore a few shader keywords only relevant at map generation time.

These keywords were causing some shaders to be skipped, causing the ‘missing
texture’ texture to be displayed.
fix potential buffer overflow for openal sfxHandle_t registrations
Changelog update.
Add functions for setting file access permissions.

Implemented on POSIX-compliant systems.
Only no-ops for Windows since I can not test.
Mark the pubkey file as owner r/w only.