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.

src/backend/port/atomics.c

@@ -21,7 +21,6 @@
 #ifdef WIN32
 #error "barriers are required (and provided) on WIN32 platforms"
 #endif
-#include <sys/types.h>
 #include <signal.h>
 #endif
 

src/backend/port/dynloader/freebsd.h

@@ -13,7 +13,6 @@
 #ifndef PORT_PROTOS_H
 #define PORT_PROTOS_H
 
-#include <sys/types.h>
 #include <nlist.h>
 #include <link.h>
 #include <dlfcn.h>

src/backend/port/dynloader/netbsd.h

@@ -14,7 +14,6 @@
 #ifndef PORT_PROTOS_H
 #define PORT_PROTOS_H
 
-#include <sys/types.h>
 #include <nlist.h>
 #include <link.h>
 #include <dlfcn.h>

src/backend/port/dynloader/openbsd.h

@@ -13,7 +13,6 @@
 #ifndef PORT_PROTOS_H
 #define PORT_PROTOS_H
 
-#include <sys/types.h>
 #include <nlist.h>
 #include <link.h>
 #include <dlfcn.h>

src/backend/port/win32/crashdump.c

@@ -39,8 +39,7 @@
 #include "postgres.h"
 
 #define WIN32_LEAN_AND_MEAN
-#include <windows.h>
-#include <string.h>
+
 /*
  * Some versions of the MS SDK contain "typedef enum { ... } ;" which the MS
  * compiler quite sanely complains about. Well done, Microsoft.