Don't allow modifying qvms or pk3s

Exception for allowing pk3s to be downloaded.
Rename FS_CheckFilenameIsNotExecutable to ..NotImmutable
Rename FS_CheckFilenameIsNotImmutable to ..IsMutable
Remove fallback r_picmap

Should have been r_picmip, but don't want that set to 1 unless user specifies it.
Merge remote-tracking branch 'ioquake3/master'

Conflicts:
	code/qcommon/files.c
Don't reset r_texture* to non-default values

These where the Q3 defaults, but doesn't really make sense to set them after crashing.
Correct TeamCount return type and team argument type
Merge remote-tracking branch 'ioquake3/master'
#5889: Do post-processing only on viewport portion of frame buffer.
Merge remote-tracking branch 'ioquake3/master'
Merge remote-tracking branch 'spearmint/master'
Allow using external zlib.h in tr_public.h