Cleanup optimizer function names and clarify code.

src/backend/optimizer/README

src/backend/optimizer/geqo/geqo_eval.c

src/backend/optimizer/geqo/geqo_main.c

src/backend/optimizer/geqo/minspantree.c

src/backend/optimizer/path/allpaths.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/path/joinrels.c

src/backend/optimizer/path/xfunc.c

src/backend/optimizer/plan/initsplan.c

src/backend/optimizer/plan/planmain.c

src/backend/optimizer/util/clauses.c

src/backend/optimizer/util/joininfo.c

src/backend/optimizer/util/relnode.c

src/backend/parser/gram.c

src/include/nodes/parsenodes.h

src/include/optimizer/clauses.h

src/include/optimizer/planmain.h