convert colvars lib to use c++ style header files

lib/colvars/colvarmodule.cpp

@@ -8,7 +8,7 @@
 // Colvars repository at GitHub.
 
 #include <sstream>
-#include <string.h>
+#include <cstring>
 
 #include "colvarmodule.h"
 #include "colvarparse.h"

lib/colvars/colvarproxy.cpp

@@ -8,7 +8,7 @@
 // Colvars repository at GitHub.
 
 #include <sstream>
-#include <string.h>
+#include <cstring>
 
 #if defined(_OPENMP)
 #include <omp.h>

lib/colvars/colvarscript.cpp

@@ -8,8 +8,7 @@
 // Colvars repository at GitHub.
 
 #include <cstdlib>
-#include <stdlib.h>
-#include <string.h>
+#include <cstring>
 
 #define COLVARSCRIPT_CPP
 #include "colvarscript.h"

lib/colvars/colvartypes.cpp

@@ -7,8 +7,8 @@
 // If you wish to distribute your changes, please submit them to the
 // Colvars repository at GitHub.
 
-#include <stdlib.h>
-#include <string.h>
+#include <cstdlib>
+#include <cstring>
 
 #include "colvarmodule.h"
 #include "colvartypes.h"