replace C-style include files with their C++ equivalents

src/USER-MISC/fix_imd.cpp

@@ -58,10 +58,10 @@ negotiate an appropriate license for such distribution."
 #include "group.h"
 #include "memory.h"
 
-#include <math.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
+#include <cmath>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #include <winsock2.h>
@@ -350,7 +350,7 @@ static void id_sort(tagint *idmap, tagint left, tagint right)
 
 /* part 1: Interactive MD (IMD) API */
 
-#include <limits.h>
+#include <climits>
 
 #if ( INT_MAX == 2147483647 )
 typedef int     int32;