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/cslib_wrap.cpp

@@ -44,7 +44,7 @@ void cslib_open_fortran(int csflag, const char *mode, const char *str,
                         const void *pcomm, void **csptr)
 {
   MPI_Comm ccomm;
-  void *pccomm = NULL;
+  void *pccomm = nullptr;
 
   if (pcomm) {
     MPI_Fint *fcomm = (MPI_Fint *) pcomm;