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

src/fix_restrain.cpp

@@ -45,9 +45,9 @@ enum{BOND,LBOUND,ANGLE,DIHEDRAL};
 
 FixRestrain::FixRestrain(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  rstyle(NULL), mult(NULL), ids(NULL), kstart(NULL), kstop(NULL),
-  deqstart(NULL), deqstop(NULL), target(NULL), cos_target(NULL),
-  sin_target(NULL)
+  rstyle(nullptr), mult(nullptr), ids(nullptr), kstart(nullptr), kstop(nullptr),
+  deqstart(nullptr), deqstop(nullptr), target(nullptr), cos_target(nullptr),
+  sin_target(nullptr)
 {
   if (narg < 4) error->all(FLERR,"Illegal fix restrain command");