T2345: Replace instances of NULL with nullptr

The following changes have been applied to src and lib folders:
regex replace: ([^"_])NULL ⇒ \1nullptr (8968 chgs in src, 1153 in lib)
Manually find/change: (void \*) nullptr ⇒ nullptr (1 case)
regex find: ".*?nullptr.*?"
  Manually ~14 cases back to "NULL" in src, ~2 in lib
  regex finds a few false positive where nullptr appears between two
  strings in a function call

lib/message/cslib/src/msg_mpi_two.cpp

@@ -52,7 +52,7 @@ void MsgMPITwo::init(char *filename)
 {
   if (client) {
     if (me == 0) {
-      FILE *fp = NULL;
+      FILE *fp = nullptr;
       while (!fp) {
         fp = fopen(filename,"r");
         if (!fp) sleep(1);