Remove useless duplicate inclusions of system header files.

c.h #includes a number of core libc header files, such as <stdio.h>.
There's no point in re-including these after having read postgres.h,
postgres_fe.h, or c.h; so remove code that did so.

While at it, also fix some places that were ignoring our standard pattern
of "include postgres[_fe].h, then system header files, then other Postgres
header files".  While there's not any great magic in doing it that way
rather than system headers last, it's silly to have just a few files
deviating from the general pattern.  (But I didn't attempt to enforce this
globally, only in files I was touching anyway.)

I'd be the first to say that this is mostly compulsive neatnik-ism,
but over time it might save enough compile cycles to be useful.