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/interfaces/ecpg/ecpglib/data.c

@@ -3,8 +3,6 @@
 #define POSTGRES_ECPG_INTERNAL
 #include "postgres_fe.h"
 
-#include <stdlib.h>
-#include <string.h>
 #include <float.h>
 #include <math.h>
 

src/interfaces/ecpg/ecpglib/execute.c

@@ -16,7 +16,6 @@
 #define POSTGRES_ECPG_INTERNAL
 #include "postgres_fe.h"
 
-#include <locale.h>
 #include <float.h>
 #include <math.h>
 

src/interfaces/ecpg/ecpglib/typename.c

@@ -3,7 +3,6 @@
 #define POSTGRES_ECPG_INTERNAL
 #include "postgres_fe.h"
 
-#include <stdlib.h>
 #include "ecpgtype.h"
 #include "ecpglib.h"
 #include "extern.h"

src/interfaces/ecpg/preproc/ecpg.c

@@ -6,7 +6,7 @@
 #include "postgres_fe.h"
 
 #include <unistd.h>
-#include <string.h>
+
 #include "getopt_long.h"
 
 #include "extern.h"

src/interfaces/ecpg/preproc/pgc.l

@@ -19,7 +19,6 @@
 #include "postgres_fe.h"
 
 #include <ctype.h>
-#include <sys/types.h>
 #include <limits.h>
 
 #include "extern.h"

src/interfaces/libpq/fe-connect.c

@@ -15,7 +15,6 @@
 
 #include "postgres_fe.h"
 
-#include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <ctype.h>

src/interfaces/libpq/libpq-int.h

@@ -25,7 +25,6 @@
 #include "libpq-events.h"
 
 #include <time.h>
-#include <sys/types.h>
 #ifndef WIN32
 #include <sys/time.h>
 #endif

src/interfaces/libpq/pthread-win32.c

@@ -12,7 +12,6 @@
 
 #include "postgres_fe.h"
 
-#include <windows.h>
 #include "pthread-win32.h"
 
 DWORD