Rename pg_plan and pg_eval to be more meaningful.

src/backend/catalog/heap.c

src/backend/catalog/pg_proc.c

src/backend/commands/_deadcode/version.c

src/backend/commands/dbcommands.c

src/backend/commands/user.c

src/backend/executor/functions.c

src/backend/executor/spi.c

src/backend/optimizer/path/xfunc.c

src/backend/rewrite/rewriteDefine.c

src/backend/tcop/postgres.c

src/include/tcop/tcopprot.h