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.
Continuing 63f32f3416a8b4f8e057dc184e8e8eae734ccc8a, libpgcommon should
depend on libpgport, but not vice versa. But wait_result_to_str() in
wait_error.c depends on pstrdup() in libpgcommon. So move exec.c and
wait_error.c from libpgport to libpgcommon. Also switch the link order
in the place that's actually used by the failing ecpg builds.
The function declarations have been left in port.h for now. That should
perhaps be separated sometime.
