simplify inclusion of system header files.

This is a mechanical clean-up of the way *.c files include
system header files.

 (1) sources under compat/, platform sha-1 implementations, and
     xdelta code are exempt from the following rules;

 (2) the first #include must be "git-compat-util.h" or one of
     our own header file that includes it first (e.g. config.h,
     builtin.h, pkt-line.h);

 (3) system headers that are included in "git-compat-util.h"
     need not be included in individual C source files.

 (4) "git-compat-util.h" does not have to include subsystem
     specific header files (e.g. expat.h).

Signed-off-by: Junio C Hamano <junkio@cox.net>

imap-send.c

@@ -24,13 +24,6 @@
 
 #include "cache.h"
 
-#include <assert.h>
-#include <netinet/in.h>
-#include <netinet/tcp.h>
-#include <arpa/inet.h>
-#include <sys/socket.h>
-#include <netdb.h>
-
 typedef struct store_conf {
 	char *name;
 	const char *path; /* should this be here? its interpretation is driver-specific */