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/common/ip.c

@@ -24,7 +24,6 @@
 #endif
 
 #include <unistd.h>
-#include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
 #include <netdb.h>

src/common/username.c

@@ -18,10 +18,8 @@
 #include "postgres_fe.h"
 #endif
 
-#include <errno.h>
 #include <pwd.h>
 #include <unistd.h>
-#include <sys/types.h>
 
 #include "common/username.h"
 

src/common/wait_error.c

@@ -21,8 +21,6 @@
 #endif
 
 #include <signal.h>
-#include <stdio.h>
-#include <string.h>
 #include <sys/wait.h>
 
 /*