engine.c: after creating gray fog palookup, make its greatest shade map to same color.

Concretely, for all 1 <= i <= 255, make graypalookup[31][i] := graypalookup[31][0].
Also document 'fogpal' in the Lunatic doc.

git-svn-id: http://svn.eduke32.com/eduke32@4456 1a8010ca-5511-0410-912e-c29ae57300e0
engine.c: extend the last workaround to all four fogpals generated at startup.

git-svn-id: http://svn.eduke32.com/eduke32@4457 1a8010ca-5511-0410-912e-c29ae57300e0