include-file cleanup

It took a little work to recompile after the include-file cleanups :-(.
You got overenthusiastic about removing #includes, apparently.
I have checked in changes for the ones that caused compile failures
or warnings here, but there may be more.

One thing that particularly disturbs me is that "config.h" was removed
from some of the files in src/backend/port/.  I had to put this back
in the ones used on my platform.  I didn't touch anything I didn't get
a warning from, but I would strongly counsel making sure that config.h
still gets included *everywhere*, even if it doesn't seem necessary.

Failing to do so may cause subtle problems due to #define symbols not
being defined where they should be or prototypes not being visible that
should be.  This is especially dangerous in platform-specific code.
        regards, tom lane